Leaders need the backing of other significant elements in society, including the defence force, to conduct a war. They can make a lot of money out of wars. Another cause of war is hegemony - when a powerful state seeks to spread its influence through military means. Germany and Japan in the second world war would be examples. Hegemony results in mutual fear.
Nations seek hegemony to safeguard and expand their sovereignty.
Other states need to protect themselves against the hegemon so they enter into alliances so they can have a stronger response. Warlike postures and actions can sometimnes have a domestic political reason. The presence of military arms is a cause of conflict - ifyou don't have them you cannot go to war obviously. The sovereign power of the state is often reflected in military acquisitions of arms. Nearly all states have armed forces, even some of the smallest and poorest states.