He gained his earliest military experiences during the Long War in Hungary, where his elder half-brother Charles (1543–1595), also a soldier of renown, held a high command in the imperial army. While his brother succumbed to an epidemic within short time, young Ernst stayed at the theatre of war for several years. In the War of the Jülich Succession he served under Archduke Leopold V of Austria, until that prince's ingratitude, real or fancied, drove him into the arms of the enemies of the House of Habsburg. Although he remained a Roman Catholic, from about 1610 he openly allied himself with the Protestant princes, and during the earlier part of the Thirty Years' War he was one of their foremost champions.