World War I started in 1914 when Austria-Hungary declared war on Serbia. Austria-Hungary was upset that a group of Serbian nationals had killed Franz Ferdinand, the next King of Austria-Hungary.
Because of the system of entangling alliances, this two-country dispute quickly escalated into a multi-country dispute. On one side was the Central Powers that consisted of Germany, Austria-Hungary, Turkey, and Bulgaria. On the other side was the Allied Powers that consisted of Great Britain, France, the Soviet Union, and later Italy, and the United States.
The Central Powers had a plan to win the war. They wanted to control the western front by defeating France and then fight the Soviet Union on the eastern front. However, the Central Powers weren’t able to accomplish this and had to fight a two-front war for a good part of World War I. The system of trench warfare that developed on the western front was a very deadly method of fighting.
The Central Powers also attacked United States ships that were heading to Great Britain and France. The Germans used the submarine. This was a weapon that was designed to strike fear into our merchant marine. The submarine would attack without warning. However, this violated our rights as a neutral nation. We had a right to trade with countries at war as long as we weren't trading war materials. The Central Powers could search our ships, but they couldn’t sink them without warning. When Germany stopped sinking our ships without warning in 1916, things calmed down for a while. However, when Germany resumed sinking our ships without warning and encouraged Mexico to attack us, we declared war and entered World War I.
The United States helped the Allies win the war. There was a great deal of concern when the Soviet Union pulled out of World War I in 1917. However, our troops were well rested, and we provided the Allies with essential supplies. When the Germans tried to break through the trenches on the western front in 1918, they were defeated and had to retreat. The Allies pursued the Germans, and they eventually surrendered.
Eventually, a peace agreement was signed between the Allies and the Central Powers. This treaty, called the Treaty of Versailles, ended the war. It was very harsh on the Central Powers, especially on Germany.