To Aquinas, a war must be just in both the reasons for going to war and how war is fought.
They attempted to justify war, and reconcile it with the Christian belief that taking a human life is wrong. This idea dates back to ancient times, but is most clearly traced to the writings of St. Just War Theory In Western tradition, there is a sense that the reasons for war must be just. In many instances, the president has used military force without declaring war. In the United States, the legal power to declare war is vested in Congress however, the president is the commander-in-chief of the military, so he or she holds power to conduct a war once it has been declared. Others have claimed that most wars today are fought for ideological reasons.
More specifically, some have argued that wars are fought primarily for economic, religious, and political reasons. It has been argued that a nation will go to war if the benefits of war are deemed to outweigh the disadvantages, and if there is a sense that there is not another mutually agreeable solution.
Nations go to war for a variety of reasons. War is generally defined as violent conflict between states or nations.