1. The Assassination of Archduke Franz Ferdinand
This was the cause for World War I. Gavrilo Princip, a Bosnian nationalist, decided to kill Franz Ferdinand, an Austrian Archduke. This happened on June 28, 1914, and WWI started on 28 July, a month after the Archduke was assassinated. The reason why he did this was because parts of Yugoslavia belonged to Austria, and he wanted their independence.
2. The sinking of the Lusitania
This happened on May 7, 1915, during WWI. The United Kingdom established a naval blockade of Germany, which meant that it couldn't use the seas at all. Germany obviously didn't like this, so it sank RMS Lusitania, a British ocean liner, as a sign of revolt.
3. The Zimmerman Telegram
This refers to the correspondence between Germany and Mexico during WWI. Germany proposed an alliance to Mexico, however, this telegram was intercepted by the British. The alliance suggested that if Mexico helps Germany, it would help it in turn to take over Texas, Arizona, and New Mexico. When the US found out about this, it declared war on Germany.
4. The Bolshevik Revolution
This refers to a series of revolutions which started in Russia in February 1917. The aim of the revolutions was to overthrow the Russian monarch because of his terrible rule of the country. In the end, the Bolsheviks won, and established the Communist Party in Russia at the time.
5. The Treaty of Versailles
This is one of the most important documents in the history of the world because it was signed in order to end WWI. The treaty was signed on 28 June, 1919, in Versailles, France. One of the most important provisions of this treaty was that Germany had to accept its responsibility for its actions during the War.