I feel that detailing the factors concerning Austria’s security against France in the age before the unification of Germany would be useful for explaining the geopolitical landscape of Enlightenment Europe.
Austria’s advanced position west of the Rhine during the Middle Ages and the Renaissance was situated on the fortress of Ensisheim, in the Sundgau. Today Ensisheim is part of France in the Haut-Rhin Department (in Alsace).
The French were still confined west of the Vosges until the Thirty Years' War (1618-1648), which they would have had to cross in order to lay siege to Ensisheim.
The easiest passage into Alsace is to the south through the Burgundian Gate, between the Jura Mountains to the south and the Vosges Mountains to the north, commanded by the fortress of Belfort. Belfort was an Austrian fortress in the Sundgau, named Beffert until 1648. The French would have had to take Belfort to enter Alsace.
Once France had taken the fortress of Ensisheim having entered Alsace after 1648, she was then able cross to the east bank of the Rhine.
Here she immediately faced the fortresses of Breisach and Freiburg in the Austrian Breisgau, today part of Germany in the state of Baden-Wirtemberg.
Once Breisach was taken the French could enter the Black Forest, where they were opposed by the numerous forts in the Austrian Vorlande such as Villingen, Stockach, Hohentwiel and, most importantly, Rottenburg on the River Neckar.
After taking Rottenburg and emerging from the Black Forest, the French were immediately faced by the flowing waters of the River Iller as it rushed down from the Alps to join the Danube farther north.
Anchoring the line of the Iller was the great fortress of Ulm, where Napoleon famously defeated Karl Mack von Leiberich in October, 1805.
Following the fall of Ulm, the French could cross the Iller and over the Swabian Alps into Bavaria.
Bavaria is a densely populated and rich area in southeastern Germany that was defended by several Bavarian forts, including Donauwörth, Ingolstadt, and the Bavarian capital of Munich, and also the Imperial Free Cities of Augsburg, Regensburg, and Nuremberg, that Austria had access to by virtue of the Habsburgs being Emperors of Germany.
The River Danube rushes from west to east across Bavaria, cutting her in two before crossing the Austrian border near Passau.
What this means in practical terms is that one was forced to operate either north or south of the river, otherwise their forces would be separated from each other by it and thereby risk defeat in detail.
The problem with concentrating on one side of the river, however, was that the enemy could shadow you from the other side.
This was the problem faced by the Swedes under Gustavus Adolphus, and most famously by the Comte de Tallard before Blenheim as the Austrians under Eugene of Savoy on the north bank marched parallel to the French on the south bank, and then darted across the river and defeated them at Blenheim in 1704 when the Anglo-Dutch force under the Duke of Marlborough arrived to reinforce them from the Rhine.
The southern side was the more populous, and the more important. But it was also the more difficult as their were several right bank tributaries of the Danube that consecutively cut across the path of an invader.
There were advantages and disadvantages to both banks. If one took the more northerly route, they could cross into Austria more easily and more quickly. On the other hand, if they took the southern path they’d find more resources to extort and if a march on Vienna failed, at least they could establish themselves in Bavaria.
If the southern half of Bavaria was chosen, once crossing the Iller the French would be faced next by the line of the Lech, centred on the city of Augsburg.
The Swedish under Gustavus Adolphus during the Thirty Years’ War managed to cross the Lech at Rain, by having their cavalry ford it farther south and getting behind the Bavarians under Tilly. This ruse succeeded in getting the Swedes into Bavaria, but they, and any potential invader, were faced with the next river line.
And this line is, of course, the Isar. The Upper Isar is strongly defended by the Bavarian capital of Munich, additionally protected from the west by the line Ammer-Lake Ammer-Amper, which joins the Isar north of Munich at Moosberg.
The river then turns gently east flowing in a northeasterly direction until it joins the Danube just south of the Bohemian Forest. The Lower Isar was commanded by the fortress of Landshut.
Though the Swedes under Gustavus briefly took Munich, they did not take Landshut and were defeated by the Austrians under Wallenstein at Alte Veste. Gustavus then abandoned crossing the Isar and moved north to save his Saxon ally, whom Wallenstein now threatened in order to, successfully, get the Swedish King out of Bavaria. As is well known, the Swedish King soon thereafter lost his life in Saxony on the field of Lützen.
Returning now to our subject, we understand that if the French managed to defeat the major field army in Bavaria and systematically reduced at least one side of the Danube, they would next be faced by the River Inn as it flows north from the Tyrol into the Danube, and the River Ilz as it flows south from the Bohemian Forest into the Danube, forming a continuous line delineating the current Austro-Bavarian border, shielding Austria behind it to the east.
The banks of Inn is the famous birthplace of Adolf Hitler, who was born on the east bank in Braunau, but the river’s main defence is further downstream, at Passau near the confluence of the Inn and Ilz with the Danube.
Once across the Inn-Ilz line subsequent to capturing Passau, the French were faced with the same problem in Austria itself as in Bavaria, having to operate either north or south of the Danube but in an even more formidable manner .
The Danube east of the Austrian frontier forms a relatively narrow valley between the Kalkalpen Alps and the Hausruck to the south, and the lower extensions of the Bohemian Forest like the Sauwald and the Weinsberger Wald to the north.
In this constricted dale along the Danube and its nearby tributaries lies the heartland of Austria, and its major population centres like Melk, Linz, St. Pölten, Steyr, Wels, Amstetten, Schärding, Brunau, Salzburg, Grein, Krems, Stockerau, Korneuburg, and of course, Vienna itself.
Thus Austria could yet defend successive river lines east of the Inn and south of the Danube. This is logical avenue of approach for to the north of these river lines, reaching almost to the left bank of the Danube, are the mountainous forests of the Mühlviertel and the Weinsberger Wald.
The north bank only opens up with flatter, easier, terrain east of Krems in the Weinviertel district northwest of Vienna, famous for its vinyards (hence the name), sort of resembling the landscape of Tuscany.
This forced enemies to the limited corridor south of the Danube and east of the Inn. Immediately east of the Inn was the Traun, anchored between Wels and Linz. Beyond that lay the Enns, anchored on Steyr. Further beyond that, lay the line of the Traisen (right bank) and Kamp (left bank) with the city of St. Pölten.
This makes the avenue of advance predictable from the west, and therefore more capable of being effectively resisted. The Bavarians attempted to come this way in 1741-1742, but were defeated by Ludwig von Khevenhüller after getting as far as St. Pölten on the line of the Traisen-Kamp, on the verge of breaking into the open plains to the east.
So one is made to understand the strategic value of the seemingly useless, from a cursory glance at the map, Habsburg Vorlande, a group of tiny enclaves in Swabia and Alsace separated from Austria's heart much farther east.
In fact they placed Austria’s forward defences well to the west of the Rhine, and securely held down the Black Forest.
So not only was the Rhine between France and Austria, but the Black Forest, the Iller, Bavaria, the Inn, and the fortresses along the Danube.
Operating in the Black Forest was hazardous for France, and it was here that Archduke Charles defeated Jean Moreau at Stockach and Ostrach during the War of the First Coalition, long before the French got anywhere near the Austrian frontier.
Bavaria was dangerous to campaign in as well, for the Austrians could retire to whichever bank of the Danube the French didn’t take, and shadow them until an opportunity arose to fall upon them, as at Blenheim.
This same strength was afforded the Austrians once in Austria itself, where the enemy was almost by default compelled to approach Vienna along the south bank.
At Aspern in 1809 the Austrians remained on the north bank while Napoleon remained on the south bank. Though Vienna had fallen, Napoleon could not win the war without defeating the Austrian field army.
He could not engage that without first crossing the Danube, which he attempted to do by using an island in the river, named Lobau, as a springboard. His initial crossing at Aspern was repulsed with massive casualties, including Marshal Lannes whose legs were blown off by an Austrian cannonball.
Though Napoleon was able successfully to cross later, at Wagram, the losses sustained by the French in the two engagements had seriously weakened him, and he was prepared for a moderate peace.
The Austrians therefore were given ample opportunity to arrest the advance of the French across southern Germany, generally they depended upon their fortresses in the Black Forest and in Alsace to break the French surge, using the time afforded to retaliate once they found a favourable opening.
Only once did this line completely fail them, and that was in 1805. In that year Napoleon stood on the Channel at Boulogne, intending to invade England. When Austria declared war he broke camp and marched southeast.
But he did not cross the Rhine in Alsace as was the traditional route into Germany, he instead crossed it much farther north after moving through Belgium. This allowed him to bypass the Black Forest entirely, and arrive at Ulm far sooner than Mack expected.
Before he realised it, Mack was trapped on the Iller with the French all around him, as Napoleon had circumvented the Iller by arriving from the north, crossing the Danube at Donauwörth and Ingolstadt east of Ulm, that is, to the rear of the Austrians.
This disaster cost Austria its entire field army in Bavaria, as Mack, realising he was cornered, surrendered all of his troops. Napoleon wasted no time in exploiting this fortuitous development by rushing through Bavaria and entering Austria.
Being unable to muster a sufficient force to meet this, the Austrians retreated with what they had into Bohemia by moving up the valley of the Morava where they sought to link with the Russians, who had crossed the Moravian Gate from Galicia.
Having thus taking Bavaria and the Austrian core cities along the Danube without a fight, Napoleon was able to give the Austro-Russians the coup de grâce at Austerlitz.
Defeat at Austerlitz was the end of Austria's participation in the War of the Third Coalition.
In this somewhat desultory manner we are once more given powerful illustration of the importance of Belgium in European geopolitical culture.
From their positions in Alsace the French were forced to cross the Upper Rhine where they had no choice but to lay siege to the Austrian held Breisach and then advance into the Black Forest and the Swabian Alps.
This consistently impeded their progress, weakened them, and wasted their time.
But in 1805 the situation was different. France had annexed Belgium, and also the Netherlands. This allowed Napoleon to cross the Rhine much farther north, follow it to the Main, up that river, and skirt across the northern limits of the Black Forest, avoiding both the forest itself and all the Austrian forces in/around it.
It also allowed him to bypass the line of the Iller. By crossing the Lower Rhine, darting southeast, and crossing the Danube in Bavaria east of Ulm, Napoleon had outflanked all the traditional Austrian defences to the west. In effect, he bagged all the Austrian forces between the Rhine and the Iller simply by moving around them to the north.
Because of this, Austria absolutely could not allow France to gain Belgium, and it’s very fortunate therefore that Austria did not recover Silesia in 1757, as that would have entailed French acquisition of Belgium.
Austria’s consequent profound weakness to a French attack directed from across the Lower Rhine bound Britain, the Netherlands, and the Germans to invest heavily in a Belgium permanently separate from France. The loss of Belgium, as we've seen, would mean the dangerous exposure of Austria on the grounds that it would nullify Austria's natural outer glacis in southwestern Germany.
The decisive elimination of the Austrian Army meant the supremacy of France in Germany and Italy by default, as it was the sole institution capable of preventing French domination of Central Europe.
Simply put, in order to prevent French hegemony, and even to ensure her own survival against French assault, Austria required that the Black Forest remain between herself and France. This is why Austria fought so doggedly for the Vorlande, and why even after 1815, when Austria accepted that her lands in Swabia were to be incorporated into Baden and Wirtemberg, she insisted on garrisoning their fortresses with her own troops.
Now one might object that the French could simply move downstream along the Rhine to Mainz and from there north of the Black Forest to take the same direction Napoleon took in the middle stages of his advance to Vienna in 1805. But there were some problems to this.
Most importantly the French line of communications would be dependent upon Strasbourg and the Lauter, and if the Austrians were in an advanced western position at Breisach and Freiburg, they could sever these lines of supply by crossing to the south of the French at Philippsburg or Landau, and trap the French inside Germany. As the Austrians could only operate north of the Main with difficulty, their ability to cut the French lines of supply from Belgium was much diminished as opposed to cutting them from Alsace.
Secondly, the Austrians would be in close proximity to the French originating on the left bank of the Rhine, and a dash by them to the north would be noticed. Accordingly, an attempt to cross the Upper Rhine between Switzerland and Mainz would be detected. The Austrians would be able to fall upon the French either during the crossing or soon after, such as at Ettlingen in 1796 and Saltzbach in 1675.
In this sense crossing farther south was preferable as the Black Forest would shield France's own left flank, with their right protected by Switzerland, as they made for the headwaters of the Danube.
While this made severing their communications with Alsace more difficult, it forced them to fight headlong into the mountains of southern Germany, where they could be halted either there or in Bavaria.
The outer defences of Austria had first been breached by France under Richelieu and Louis XIV. With the Treaties of Westphalia, the Vosges had been successfully surmounted and the Sundgau was annexed by France. The French soon moved downstream to capture the great bridges over the Rhine at Strasbourg, which form France’s eastern frontier today.
But after this coup the French went no further. The fortress of Breisach east of the Rhine was oftentimes captured, and sometimes formally annexed by France, but every time she was forced to relinquish it. Attempts at invading Germany from this direction to strike Austria were always barren.
That perforce turned Europe’s attention away to the north, to Belgium. This became the crux in nearly all of the numerous coalition wars against France, such as the War of the Austrian Succession where Louis XV abandoned trying to cross the Upper Rhine around the vicinity of the Black Forest, and instead turned his attention towards the Southern Netherlands. At this time, if not earlier, the French realised their gateway into Germany was through Belgium and not through the Black Forest.
The result was the intensification of French efforts to acquire Belgium, which they had always covetted anyway. Most people know, thanks to the world wars, that Belgium is the Porta Gallia, Germany’s door through which she could enter France. But few bear in mind that Belgium was equally France’s window into Germany.
As an extension on this, after the unification of Germany France's problem was obviously no longer knocking out the Austrian Empire situated in the southeast of Germany.
But the same factors mentioned still held true. France would find it difficult to invade Germany through either the Black Forest, or by striking north into the Palatinate Forest where she would slam into the Hunsrück and the other Rhenish massifs beyond it.
France could, of course, invade either direction. But she would have only weakened herself without inflicting much damage on Germany. Just as seizing Bavaria and Swabia was not a fatal blow to Austria, it wasn't a fatal blow to national Germany either.
But the united Germany was in many respects in a worse position than Austria. It was, and is, even more exposed than Austria to a move through Belgium.
This is because, whereas France could cross the Lower Rhine to slip around the Black Forest to approach the Danube valley from the north, where it was weak, instead of the west, where it wasn't, France wouldn't even need to reach the Danube at all in the case of Germany.
Belgium would give France access to the exposed Rhineland. This is Germany's Achilles' Heel as the vast majority of its industry and a large proportion of its population are centred in the Rhineland, especially along the valley of the River Ruhr.
All France would have to do is merely enter the Rhineland, which is just a matter of leaping over the Belgo-German border, and she would cripple Germany. Perhaps fatally.
For this reason, for example, the Germans could not take the risk of France entering the First World War belatedly on Russia's side.
To briefly explain, the Germans would be unlikely to knock Russia out by occupying Poland or defending themselves. They would necessarily have to strike deeper into Russia.
The farther they drove into Russia, the greater the distance between their armies and the Rhineland would become.
Therefore even overwhelming German successes against Russia would tend to decrease Germany's security.
If the French struck at western Germany through Belgium, they could easily reach the Rhineland before the Germans could redeploy to stop them.
Such a development would leave Germany in dire straits. The greater part of its industry would be in French hands. Russia would have yet to be defeated.
Consequently Germany chose to attack France in 1914 because not doing somay have been potentially catastrophic.
In line with these considerations, Alsace was taken by Germany not so much as to secure Germany from attack, the Black Forest and the Hunsrück served that purpose already, it was to make France more vulnerable to attack.
Germany's calculation was that the precarious position of their eastern frontier would force the French to remain opposite Alsace and prevent them from exploiting Belgium themselves.
Examination of historical precedents and common military sense supported this conclusion. When the French entered the Thirty Years' War in 1635, they immediately invaded the Spanish Netherlands. They were called away when an Austrian army under Matthias Gallas suddenly crossed the Rhine into Alsace and passed through the Burgundian Gate.
During the Franco-Dutch War Raimondo Monteccuccoli obliged the French to withdraw from Holland and thereby saved the Dutch Republic by leading an Austrian army over the Rhine into Alsace.
This proved a handicap in the Nine Years' War too. The Elector of Bavaria rescued Namur and Charleroi in 1690 by throwing his forces over the Rhine into Alsace. The Margrave of Baden saved Mons in 1691 with the same method, and his presence in Alsace in 1695 allowed the Dutch to retake Namur (which France had taken in 1692).
During the War of the Austrian Succession Otto von Traun called Louis XV and his main army from Belgium by forcing the Lines of Wissembourg and breaking in to Alsace.
And finally the appearance of Dagobert von Wurmser during the War of the First Coalition in Alsace allowed Austria to regain the Netherlands from France, until Lazare Hoche threw the Austrians back over the Rhine in December of 1793, which allowed the French to powerfully reinforce Jean-Baptiste Jourdan and Jean-Baptiste Kléber who crushed Josias von Coburg at Fleurus in June of 1794, definitively winning in Belgium.
Bismarck doubtless based his acquisition of Alsace on these considerations. And this absolutely worked as events were to show. The French foreswore any possibility of striking the German Rhineland through Belgium, both in order to win Britain to her cause and, probably more importantly, because her position along the German border was so weak that the French initially deployed the bulk of their forces opposite the Vosges. But we'll leave this theme for another day.
The map shows the traditional route of advance from France to Vienna in red, across the Upper Rhine and over the Black Forest to reach the source of the Danube, and thereafter follow the river either on the north or south bank until reaching Austria. In the event the French were never able to make it as far as Vienna using this route, though they threatened to during the War of the Spanish Succession and War of the Austrian Succession.
In yellow is the route Napoleon took in 1805, which circumvented the Austrian defences in southwestern Germany and succeeded in getting between them and their base farther east. The French had been unable to use this route previously on account of their inability to successfully conquer Belgium. Only once before, towards the end of the War of the Austrian Succession, had the French actually secured Belgium. So when Austria ceded it to France by the terms of Campo Formio in 1797, Napoleon was left with an entirely unique opportunity that none of his predecessors had enjoyed. He certainly did not waste it. And this illustrated to the Austrians that their own security was bound up with Belgium, a factor they seemed to have occasionally overlooked.
- Kaiser
