Both WWI and WWII trace directly back to the assassination of Archduke Ferdinand on June 28, 1914. Yes, many of the European powers may have been looking for an excuse to start a war, but this was the event that caused it, something odd and unexpected that started Europe down that road.
Has any dated single act had as much effect on the world? The Austro-Hungarian Empire was destroyed. The Ottoman Empire was destroyed. The Communists took over Russia. Much of the world’s boundaries were redrawn by the French and British. Numerous technologies, especially the airplane, were greatly advanced. The Germans, being banned from all other tools of war, started investigating rockets. American money took over the world’s financial structure. The worldwide depression of the 1930s followed the financial ruin of Europe.
And WWI was absolutely the cause of WWII. Many people call them a common war with an interregnum. Hundreds of millions died as a result of that bullet, hundreds of millions had their lives completely upended, hundreds of millions found themselves in different countries bereft and bewildered. Nothing comparable in scope and size exists in history.