What causes war?

Anger starts war. Generally war is the result of a national entity wishing to improve the standard of living for its people. A major second cause is when a nation perceives a possible reduction in a current standard of living and fights to protect what it already has.

Britain for example, fought myriad wars all over the globe in conquest of other peoples. These were mostly wars of imperialism, where the local population was subverted to the will of the English. Ireland, India, South Africa, Australia, Ghana, etc. all suffered as the British advanced. Once in power the British would establish laws that forced the locals to produce goods for the pleasure of English industry, at less than market rates. Meanwhile English manufactured goods were sold at inflated prices to these same people. Thus the colonial peoples all subsidized the English standard of living for centuries.

Fighting to prevent a reduction in living standards was the main reason the English entered both World Wars. Both wars saw the Germans advance into the Low Countries. The British saw this as a threat to their domination of the North Sea commercial district and fought to protect their position.

The English have fought more of these type wars than any other nation so they are a great example. But other nations have done the same. Russia colonized Estonia, Ukraine, and Central Asia in similar fashion. Germany attempted to do so in Poland and the Balkans. The French pretty well followed the English pattern. Japan tried to secure a few colonies in East Asia, similar to the English empire. The USA has fought numerous wars to protect its standard of living including two in recent times in the Middle East.

All nations look to their own self interest in the final analysis. None will willingly take a reduced standard of living without a fight. It is quite likely that the future will produce several major wars over oil, as this vital substance reaches its peak production and eventual decline. These wars by necessity will include the USA and others such as Japan, China, Russia, India, and the EU. All will fight to protect their peoples standard of living.