1) Alliances: When two countries made an alliance to keep each other protected from a country, the country decided to do the same thing and eventually either one of them declared war.
2) Imperialism: When a larger country took over a smaller country, sometimes the smaller one would try fighting back. Sometimes countries would fight over a country they both want to take over.
3) Nationalism: The head of the country would love his country and hate to see it fall, so when things get rough (Say poverty or lack of food/materials) A lot of times they would go scavenging through someone else's territory (usually smaller) to get the materials they needed so their country could thrive. This could also help to start a war.
4) Militarism: A lot of countries had this type of government and a lot of times it soon became a competition between land and seas, which also lead to fights and later on, war.
