Following the devastating defeat at Austerlitz in 1805 the Austrian army was in disarray and morale very low. It seemed that Napoleon was undefeatable in Europe, the Russians had retreated eastwards and at the treaty of Pressburg in December 1805 the Austrians were forced to pay a high price for their defeat. As well as huge amounts of money Austria lost a great deal of land both the Kingdom of Italy and his ally Bavaria, such harsh conditions were to guarantee that Austria would go to war again against the French Emperor.
In 1807 the French invaded Portugal and what was to become Napoleon's open wound, the Peninsular war began. By 1808 the French were at war with Spain as well and by mid 1808. the British under Sir Arthur Wellesley had driven the French out of Portugal. The myth of French invincibility had been broken and more importantly the Spanish war was starting to draw in more and more of Napoleon's troops. Securing Russian neutrality, Napoleon left for Spain with 100,000 of his best troops from Germany. The Austrians that supported a return to war saw that their time had come. On 23rd December 1808 the war began with the Austrians planning an offensive operation against the French and their German allies in Bohemia and Bavaria with lesser operations in northern Italy, Poland and Dalmatia.
Even as the campaign began, Archduke Charles, supreme commander of the Austrian forces, had severe concerns. He felt the Army was not yet ready for such a campaign and short comings in supply and organisation soon started to become apparent as the Army started to deploy in march 1809. Meanwhile Napoleon's spy network had warned him of the Austrian preparations for an attack and he started to prepare. He warned his German allies to begin mobilising and ordered the current batch of conscripts to be rushed through training and called up others before their time. He had already done this once and the effects of this were to have long-term consequences. By these measures Napoleon created 2 Corps, (II and IV) and scraping together some reserves from other theatres of war, Napoleon managed to field 170,000 men in this area to meet the Austrian attack. Archduke Charles managed to muster about 127,000 for his main army with around 50,000 reserves.
In the early hours of 10th April the Austrian army began its advance into Bavaria. For the first few days the only enemy was the weather, which was awful. Extreme cold and snow and rain slowed the army and affected morale and with supplies getting bogged down typhoid started to break out. On the 16th the weather improved and the two armies clashed at the battle of Landshut where Bavarian forces were defending the river crossing against the Austrians. The Austrians did well and after a day's fighting had captured the town and the crossing over the river Isar, morale among the Austrian troops soared and the Archduke had been pleased with how well his light troops had performed.
At 5am the next morning Napoleon arrived to take command of the somewhat confused French forces - things were about to change. Charles slowly gathered his forces as they crossed the Isar from lower to upper Bavaria, he was unclear of the French plan but news soon reached him that Davout with 30,000 of the French had become isolated from the main French army and was deployed south of Regensburg. Charles saw a clear chance to destroy a good part of the French forces before they could rejoin the main body.
This led to the battle of Teugn-Hausen (known by the French as the battle of Thann) on 19th April 1809. The fighting took place in a densely wooded area with a variety of hills and ridges. The fighting went badly for the Austrians and the French succeeded in driving them out of the woods and holding the villages using artillery. Charles’ plan to intercept and destroy Davout was in tatters. Austrian morale, as always fragile, dropped again after this defeat. Napoleon had his poor opinion of the Austrians reinforced and on the 19th prepared to go on the attack. The 20th April saw the battle of Abensburg. Napoleon first gave a rousing speech to his Bavarian allies encouraging them to take revenge on their old enemies the Austrians with the points of their bayonets. The battle itself was made up of many sweeping engagements in which the Austrians were driven back and quickly divided up at little cost to the French. Napoleon was now convinced the morning would see him destroy the retreating Austrian army. Forced back to Landshut the Austrians were forced onto the defensive as their had not been time to destroy the bridge over the Isar and the 21st April saw the second battle of Landshut.
French infantry eventually took the bridge and the fighting became confused as Bavarian cavalry followed the French infantry into the town. The day cost the Austrians about 8,000 men and large amounts of equipment. Napoleon had now over extended himself believing he faced the main Austrian Army he found out that Regensburg had fallen to the main Austrian army. As Napoleon had believed he was driving the main Austrian army before him at Landshut, Davout was facing the main army at the first day of the battle of Eggmuhl. The Archduke Charles was in far from an ideal position, his main army although having captured Regensburg was now bogged down against one French army while a second approached with Napoleon from the south east, the only escape route over the Isar for the Austrians, Charles still believed that Napoleon was with the army he faced not with the force threatening his rear. The following battle was hard fought and resulted in Charles’ army retreating eastwards spilt into 2 separate sections, but neither had been crushed as Napoleon had expected. As the Austrians fled towards Vienna Napoleon expected them to reform and give battle but they did not. On 13th May Napoleon captured an empty Vienna after a light bombardment and expected the Austrians to continue to flee to Moravia as had happened in 1805. Archduke Charles had other plans and Napoleon was to pay the price for underestimating him when the Austrians gave him a bloody defeat at the battle of Aspern-Essling on 21-22 May.