The most common cause of war is desire for ideological change, followed by separatist and independence concerns, and then resources and territory. Determining an exact cause for war is difficult due to the many factors at work and differences between the stated aims of the combatants and their actions.
Clausewitz said that war is simply the continuation of politics by other means. By this, he meant that the same goals that drive leaders in peacetime drive them to war when other avenues have failed to provide the desired results. These political disputes historically have been over economic and demographic factors, as with the Donbass War, ongoing as of 2015.
International Relations theories seek to expose the causes of relations and interactions between countries, including warfare. Under the liberal school of thought in International Relations theory, it is primarily ideological concerns, or differences in governing philosophy, that drive war, not economic or demographic concerns. The liberal school of thought points to the rarity of war between liberal democracies as an example of evidence for this idea.
Other schools of thought in international relations theory hold different opinions on the nature of war and international relations. Realist schools agree that real, practical concerns like economics and demographics control international relations, whereas constructivist theories point towards the cultural and symbolic history of countries to explain relations between them.