As a teen-aged boy, he had gone on the Second Crusade, but now he had a chance to lead one himself. With Germany quiet and Italy arranged as best he could manage, Frederick answered the crusading call eagerly. Frederick took the cross publicly in March 1188. There was the usual long process of actually assembling an army and departing, so he did not set out until May 1189. His was probably the largest single crusading army ever to march.
Medieval estimates are always exaggerated, but the force was certainly in the tens of thousands.
The army took the traditional overland route down the Danube River and across the Balkans to Constantinople. The passage was complicated by the fact that the Byzantine emperor, Isaac Angelus, had a secret arrangement with Saladin to delay Barbarossa. Saladin had heard wild stories about the German emperor who was supposedly marching with a million men. His own men were skittish at the threat and Saladin wished to delay the day of reckoning as long as possible. Isaac Angelus did not slow up the Germans by much, but he did succeed in irritating Frederick to no end.