Why do countries from ancient times to modern times have to fight?

It is because monarchs of various countries are somewhat satisfied with the status quo after obtaining their existing achievements, territories and money, but most ambitious people will be prepared for war and spend most or more money on the existing national economy to satisfy their personal desires, while others covet other countries' resources, topography and other factors. In the middle and late stage of a country's development, there will always be a situation of population explosion and housing difficulties. War is usually used to relieve stress. These are just excuses for you, mainly depending on how we look at ourselves. Don't be too good to yourself. If we really wait until the war to change our lives, we will definitely be duty-bound to support it.