In Japan why do they have war?
In Article 9 of the Japanese constitution, it claims that the
Japanese have renounced war. This constitution was written after
World War II, and so although they have participated in wars in the
past, such as the Russo-Japanese War or Sino-Japanese War, it does
not mean that they are in war currently, or will be in a war in the
future.
If you are asking why Japan engages in war, the answer is the
same as why any country would engage in war -- territorial
expansion, imperialism, colonization, racism, etc.