Marcu had his first military engagements in the 1600 war for Moldavia, when he was chased out by the Polish army. He fled to Transylvania before 1601, and, after Michael the Brave's killing, rallied with the Holy Roman Empire, which was fighting a Long War against the Ottoman Empire. By 1602, he was given a small command function in the Imperial Army, under Giorgio Basta, and saw action at Teișani, helping to defeat the Crimean Tatars. He then renounced plans to obtain the Wallachian throne, ceding it to another Habsburg favorite, Radu Șerban. His subsequent attempts to invade Moldavia from Transylvania were curbed by the Bocskai uprising, during which he also lost control over his Transylvanian estates. He lived for several years in Austrian Bohemia at the court of Sigismund Báthory.