There were many causes of the war, each contributing to the great buildup of tension in Europe. One of these buildups was the fight for land in Africa, which Germany, Britain, and France fought for. Another was the tensions from alliances, which caused many of the countries originally not in the war to join the war. The third reason was that when the Archduke went to Serbia, he was assassinated, causing an uproar and the declaration of war.

Just like the cause of the war, there were many various reasons the U.S. joined the war. One of the reasons was the sinking of the Lusitania, though this wasn't the main reason, as it occurred a good two years before we joined the war. The main reason was the Zimmerman Telegram. This was a telegram originally sent to Mexico but was intercepted by the British, who gave it to America. It asked Mexico for help should the U.S. join the war, in exchange for American land. This forced the U.S. to declare war on Germany.

It did, indirectly. Because of the Treaty of Versailles, Germany was burdened with huge military and money issues, making them bitter. When Hitler came along, he used this bitterness to start the second world war.