The war would be determined on the continent , and the major land operations that sealed the swift French victory involved Participation the Ulm Campaign , a large wheeling manoeuvre by the Grande Armée lasting from late August to mid-October 1805 that captured Conquering an entire Austrian army , and the decisive French victory over a combined Cause_to_amalgamate Russo-Austrian force under Tsar Alexander I at the Battle of Austerlitz in early December .