Question 1:
Germany resumed unrestricted submarine warfare.The Zimmermann Telegram revealed a German attempt to gain Mexico as an ally against the United States.
Question 2:
Wilson's plan for world peace was called his "14 Points." It failed because the Treaty of Versailles did not implement most of his ideas, and the US Senate did not ratify the Treaty of Versailles because of the League of Nations (which was Wilson's idea).
Context/detail for question 1:
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.
Germany managed to stave off American entry into the war at the time by pledging to stop submarine attacks. But two years later, the Germans resumed such attacks. This inflamed public opinion against Germany once again and poised America to go to war.
In addition, British intelligence 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 Germans' submarine warfare and the Zimmermann Telegram.
Context/detail for question 2:
President Woodrow Wilson thought that some of the very things that were adopted in the Treaty of Versailles could lead to another war -- and they did. The Treaty of Versailles included various ideas that went against Wilson's plans for peace. The treaty was very punitive towards Germany. Germany was forced to admit responsibility for causing the Great War (World War I). We now call that "the war guilt" clause of the treaty. The German military had major restrictions imposed on it -- it had to be a volunteer military only, of no more than 100,000 men, and they could not have an air force. Germany also was forced to pay large reparation payments to the Allies (who opposed Germany in the war).
US President Wilson had come into the Paris Peace Conference with other views. Wilson had set forth his views in a speech he delivered to Congress in January, 1918, his "14 Points" speech. The 14 Points were his proposals for how to end the Great War (World War I) and establish international peace. Especially key to his ideas was point #14 - the establishment of an international organization to promote and preserve peace.
The Treaty of Versailles did adopt Wilson's 14th point, which led to the formation of the League of Nations. But back home in the US, Republicans in the Senate feared that commitment to a League of Nations could commit the US to future wars that were not directly related to US national security. Because of its objections to membership in the League of Nations, the United States Senate refused to ratify the Treaty of Versailles. Without the USA in the League of Nations, that organization was weakened and did not have much success.
