Heinrich served under Archduke Charles in Italy during the War of the Third Coalition. The original organization of the "Armee von Italien" called for Reuss to command an eight-battalion division. But at the Battle of Caldiero on 29–31 October 1805, Charles gave him command of the left wing. Reuss played a prominent role in the fighting, commanding Johann Kalnássy's brigade of eight line infantry battalions, Heironymus Colloredo-Mansfeld's brigade of five grenadier battalions, and the "Archduke Charles" Uhlan Regiment # 3. The fog lifted around 11 am on 30 October and Reuss' troops were immediately assaulted by Guillaume Philibert Duhesme's division. Caldiero village, held by his troops, changed hands several times during the day, as Duhesme attacked and Reuss counterattacked. The day ended with Caldiero in French hands, but the Austrian line intact. On 31 October, Reuss repelled a French probe of the Austrian left flank. The next day, Charles withdrew to the east and no more major actions occurred in the campaign.