Frederick II's renown was based above all on his ability as a military leader. In Clausewitz's great work On War he appears alongside Julius Caesar, Gustavus Adolphus, and Charles XII exclusively as a soldier and commander. Since Charles XII was more brave than skilful, more active than clever, more a slave to his passions than was good for him, according to Frederick one should only follow his example with caution. Thus Charles XII, whom Frederick secretly admired but who also represented a warning of the possibility of his own downfall, failed to pass the ultimate test of great military leadership. Unlike Charles XII, a military role was not a natural one for Frederick. Rather, his harsh upbringing forced it upon him and perhaps awakened hidden instincts in him. One of his first acts as King was to assume the full authority of a ruler, which precluded any competitive authorities from functioning.