Germany's Dissatisfaction

A U-534 used during WWII
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During World War I there were two opposing sides, the Allies and the Central Powers. A military strategy that England had was to make a blockade, when ships block or make a line to prevent a country from importing or exporting goods.. Because of this, Germany was upset with England. Germany couldn't import or export weapons, military supplies, or even food. 

Germany was also upset with the United States. Since the United States couldn't send in goods to Germany and they didn't challenge the British Blockade because these two countries were at peace with each other and the U.S. didnt want to join the war, there for they didn't challenge the British the Germans were mad with the U.S. The U.S. then couldn't import foodstuffs, or fertilizer for crops. This led to famine in 1917 where about 750,000 Germans starved to death because of the blockade. 

President Woodrow Wilson
Source.

Germany felt the need to get rid of the blockade themselves, therefore they used U-boats, (German submarine), to sink the ships blocking them from the rest of the world. The Germans would sink many British ships claiming their ammunition, or bullets/rounds, or other weapons. However those ships were not military ships, they had both British and American civilians inside. On May 7, 1915, a German U-boat sank the British liner, the Lusitania. They claimed it contained ammunition, when it really claimed more than 1,000 civilians. Of the 1,198 civilians, 128 of them were American citizens. This lead to the American public being against the Central Powers. 

Woodrow Wilson, a president of the United States, wanted, " a peace without victory... a peace between equals." He basically didn't want war with Germany or the other countries from harming each other. One of Wilsons goals was to have all countries join a "league for peace" to extend democracy, keep the freedom of the seas, and reduce weapons. In other words he wanted a "League of Nations" however that did not do anything to stop World War II from happening, and even after world war two the League of Nations had very little power and influence.