alternatehistory.com

After the seemingly peaceful 1808 in Europe, Austria struck Bavaria on 10 April 1809 with an invasion and encircled the Bavarian Army, which was forced to surrender after a devastating breakout. The corps of Davout were trapped in the city of Regensburg and were nearly lost, but were relieved by a manoeuvre of the Grand Army from the French North Sea coast to Bavaria, which reached Ulm on 28 April and engaged the Austrians a second time there, until being defeated and forced to withdraw with many soldiers captured. The Grand Army then continued to recapture Bavaria and its army over May.
By 1 June, the Grand Army crossed the Inn River into Austria and entered Vienna by 20 June. The battle of Aspern Essling occurred a week later and inflicted heavy casualties on the Grand Army before being stopped by Napoleon's intervention and counterattack. After heavy losses on both sides, the battle was over. But on 5 August, Napoleon was sufficiently reinforced to continue the offensive towards Wagram, with the 1st day's attacks costing him heavily in soldiers. The next day, an un-coordinated Austrian counteroffensive destroyed the Austrian Army and after its surrender, the Austrian Empire was stripped of its Balkan territories as they were divided between a new Hungarian nation, Dalmatia and Poland.