World War Two is usually dated from Nazi Germany's invasion of Poland in 1939 but this was only when Germany crossed the line set by Great Britain and her European allies. In fact much of the world had been in turmoil for some time before that.
In March 1936 Germany had begun build up a military presence in the Rhinelands (these had been made a demilitarisation zone in the Treaty of Versailles following World War One), followed by annexation of the Sudetenland (German speaking areas of Czechoslovakia) and Austria in 1938. Similarly in 1936 following a number of skirmishes in previous years Italy launched an invasion and occupied Abyssinia. Alienated by the league of Nations' response after this, Italy turned to an alliance with Nazi Germany. In Asia things were no more settled with Japan and Imperial China having been at war since the Japanese invasion of Manchuria in 1931. Then, of course, came the calamity that was World War Two. We've been no better since 1945 with the Korean War, Vietnam War and multiple wars in the Middle East and Afghanistan to name only a few.
You'd have thought we'd learn, wouldn't you, but we seem to keep making the same mistakes over and over, driven by fear, ideology, desire to expand territory from population increase or simple greed. It's depressing. Still I live in hope. We have the ability to change and make a better world. For the sake of humanity, I hope we do.