The assassination of Archduke Ferdinand was not the 'cause' of World War I.
His killing was the spark.
The North did not start a war with the South to free slaves in the American Civil War. That issue was a rallying point.
So, what about the Second World War? Ignore that Japan and China fought on and off for a full eight years before the invasion of Poland by Germany. Officially, Japan and China entered war over two years before September, 1939. Consider instead that on the day Hitler invaded Poland, he had been in power for over six years. The path he had taken, from being a strange little man with strange ideas, moving up in station through shrewd political maneuvering, to the manipulation of the government, banks, and people to secure total control over Germany, is the subject of many books and films.
Therefore, short of creating the longest blog post in history to detail this complicated path, I choose to begin with the opening salvo, the event that spurred declarations:
Yes, there were hints and clues leading up to this day. Entire nations had already been gobbled up by Germany (Austria and Czechoslovakia, both in 1938), and for various reasons, the allies held talks, made demands they had no intention of backing up with force, and ultimately, looked away.
Perhaps then, we get to the heart of the matter as to why we define this moment as the event that began the war. War, that most terrible of human endeavors, like many things in our lives, would be somehow better if they were simple. Simplicity is comforting. It's easy to understand a grab for land, or oil, or steel, a stretching out for 'breathing space' or 'growing room'. Wars begin at specific points in time. They end the same way. They end with defeat, or, preferably for us, in total victory.
This one started with a 'final straw'.