Austrian forces invaded Bavaria on 9 April without a formal declaration of war. Although the French were taken by surprise, victories at Abensberg and Eckmühl restored their position. Napoleon advanced to capture Vienna but he crossed the Danube too early, not anticipating the proximity of the Austrians who inflicted his biggest defeat to date at the battle of Aspern-Essling which cost him 20,000 men. A period of regrouping followed and when Napoleon crossed again in July he brought an army of 190,000, 30,000 more men than the Austrians. The Austrians were defeated and the October Treaty of Schönbrunn (also known as the Treaty of Vienna) saw them cede Illyria to the French, pay a large indemnity, and join the Continental System, a French-led coalition which had an embargo on trade with Britain.

See Rory Muir, Britain and the Defeat of Napoleon, 1807-1815 (New Haven and London: Yale University Press, 1996), pp. 91-92.