Arranged in chronological order, with dates:
The assassination of Franz Ferdinand - June, 1914Germany invaded Belgium - August, 1914The sinking of the Lusitania - May, 1915The Sussex pledge - May, 1916The Zimmerman telegraph - sent in January, 1917US declaration of war on Germany and Central Powers - April, 1917
Explanation/details:
When Austrian Archduke Franz Ferdinand was assassinated in Sarajevo in 1914 by Serbian nationalists, the reactions by the Austrian and Serbian governments kicked into gear various alliances that had been made. Russia stood ready to protect Serbia. Germany sided with Austria over against Russia. Before long the whole affair plunged Europe into war, pulling the other nations into the conflict in support of prewar alliances they had made.The military plans laid before World War I presupposed a major war between the countries which were tied together with alliances. Because the Triple Entente had Britain, France and Russia as allies, Germany thought if a war began it would need to fight on two fronts -- west and east. So German Field Marshall Alfred von Schlieffen drew up war plans that said attack France first, quickly, and then hold that territory while deploying forces to contend with Russia in the east. So when Germany declared war on Russia in 1914, the first thing it did was to invade Belgium on its way to atacking France. Thus the war spread and became instantly a more global conflict.Public outrage in the US against the Germans swept the nation following the sinking of the British ocean liner, Lusitania, which had happened in May, 1915. When a German U-boat (submarine) sank the Lusitania, over 1,000 persons were killed, including more than 100 Americans. The passenger liner was targeted by the Germans because they suspected weapons were being shipped to Britain in the cargo hold of the ship. After the Sussex, a French passenger ferry, was sunk in 1916, the US demanded and got a promise from Germany to stop their unrestricted submarine attacks. Germany managed to stave off American entry into the war at that time by the Sussex Pledge. But two years, later, beginning February 1, 1917, the Germans resumed such attacks. This poised America to go to war. British intellegence intercepted and decoded a telegram (known as the "Zimmermann Telegram") that showed Germany was trying to secure Mexico as an ally against the United States. The revelation of this plan inflamed feelings in the United States. The telegram was sent in January, 1917. In March, 1917, German Foreign Secretary Arthur Zimmermann admitted the telegram was, in fact, sent by his office. In April, 1917, the US declared war on Germany in response to the German's submarine warfare and the Zimmermann Telegram.
