After four years on the sidelines, Austria sought another war with France to avenge its recent defeats. Austria could not count on Russian support because the latter was at war with Britain, Sweden, and the Ottoman Empire in 1809. Although Archduke Charles warned that the Austrians were not ready for another showdown with Napoleon, a stance that landed him in the so-called "peace party", he did not want to see the army demobilized either. On 8 February 1809, the advocates for war finally succeeded when the Imperial Government secretly decided on another confrontation against the French.
Napoleon arrived at Donauwörth on 17 April to find the Grande Armée in a dangerous position, with its two wings separated by 120 km (75 mi) and joined together by a thin cordon of Bavarian troops. Charles pressed the left wing of the French army and hurled his men towards the III Corps of Marshal Davout. In response, Napoleon came up with a plan to cut off the Austrians in the celebrated Landshut Maneuver.
By 17 May, the main Austrian army under Charles had arrived on the Marchfeld. Charles kept the bulk of his troops several kilometres away from the river bank in hopes of concentrating them at the point where Napoleon decided to cross.
On 21 May, the French made their first major effort to cross the Danube, precipitating the Battle of Aspern-Essling. The Austrians enjoyed a comfortable numerical superiority over the French throughout the battle. On the first day, Charles disposed of 110,000 soldiers against only 31,000 commanded by Napoleon. By the second day, reinforcements had boosted French numbers up to 70,000. It was the first defeat Napoleon suffered in a major set-piece battle, and it caused excitement throughout many parts of Europe because it proved that he could be beaten on the battlefield.
In the ensuing Battle of Wagram, which also lasted two days, Napoleon commanded his forces in what was the largest battle of his career up until then. Napoleon finished off the battle with a concentrated central thrust that punctured a hole in the Austrian army and forced Charles to retreat.
In the Kingdom of Holland, the British launched the Walcheren Campaign to open up a second front in the war and to relieve the pressure on the Austrians. The British army only landed at Walcheren on 30 July, by which point the Austrians had already been defeated.