Military conflict took place during every year of the 20th Century. There were only short periods of time that the world was free of war. The total number of deaths caused by war during the 20th Century has been estimated at 187 million and is probably higher.
The Second World War was the most destructive conflict in human history. Years of international tension and aggressive expansion by Fascist Italy and Nazi Germany culminated in the German invasion of Poland on 1 September 1939. Britain and France declared war on Germany two days later.