What lessons from World War I and World War II are most relevant to today’s world?
The first and second world wars saw a transformation in the nature of war. Before 1914, war was primarily an armed conflict between military groups of nations and empires; armies or armed groups fought similarly organised and uniformed soldiers of other nations and empires. The objective was for one armed group to put its force in the field and to use violence to persuade the other side to surrender. When one side gave up, war came to an end. The political outcome of war was straightforward: to give political form to military victory.
In WWI, the interwar years and WWII, what happened was that war mutated. While it still was an armed conflict between national or imperial groups, it evolved into a different kind of conflict, in which the central armed action was violence against civilians.
The primary example of this in WWI is the Armenian Genocide of 1915 when the targets of Turkish military and paramilitary forces were Armenian civilians. After the end of the first world war, the Russian Revolution turned into a vast civil war, which degenerated into a war against everybody. In other words, there was no distinction in the Russian Revolution and the civil war that followed between attacking civilians and attacking soldiers. Before 1914, there was a clear distinction in military codes, and in humanitarian law, between attacking soldiers and attacking civilians. But that distinction was blurred during WWI and then erased afterwards.
In 1919, a war between Greece and Turkey ended in a treaty in 1923. That treaty stipulated the forced expulsion of 1.5 million Greek Orthodox civilian inhabitants of Turkey, who had to go to Greece because of their religion. Meanwhile, 500,000 Muslims who were living in Greece, because of their religion had to leave their homes, too. A peace treaty rested on ethnic cleansing.