The United States entered WWI for two main reasons - the sinking of the Lusitania and the Zimmerman Telegram. Of course, the U.S. also entered the war to assist its allies as well as fight for noble causes such as democracy and freedom, but these were not the main reasons that ultimately led to the U.S. entering the war. Some Americans were in favor of the U.S. getting involved, but many were not and wanted the U.S. to remain neutral. As we know, that didn't exactly happen.
The United States entered WWI near the end of the war. Germany had begun unrestricted naval warfare, meaning that the Germans were sinking any and all non-German ships regardless of who was on them. On May 7, 1915, Germany sunk the Lusitania, a British passenger ship. 1,200 people died as a result, 128 of which were Americans. This enraged Americans and they demanded that Germany let ships pass in peace.
The "final straw" was the Zimmerman Telegram. Zimmerman was a German official. He sent a message to Mexican officials saying that if they would help Germany win the war, Germany would help Mexico regain land that they had lost to the United States years ago. U.S. officials intercepted the telegram. While not directly declaring war on the United States, the overall message was that Germany and Mexico would declare war on the U.S. if they won. Obviously, that wasn't ideal for the U.S., so President Wilson declared war on Germany and assisted Britain and its allies with winning the war against Germany.
While many Americans felt that the U.S. had no business being involved in a European war, it is unknown what might have happened if Germany had won and then attacked the United States. The American forces played a big part in helping to end WWI, and not only European history but American history, too, might have been different if the U.S. had not entered WWI.
American involvement in World War I did not happen until towards the end of the war. The United States first intended to stay out of the war completely, but, as history tells us, that did not happen.
The first American troops arrived in Paris in the summer of 1917. It is said that American troops were driven by a hatred of autocracy and a desire to make the world a freer and better place. One of the biggest reasons that America entered the war was when a German U-boat sank the British liner Lusitania, killing about 1,200 people, including 129 Americans in May of 1915.
Another large reason that America entered the war, and the reason that pushed them over the edge which caused them to enter the war was the Zimmermann Telegram, which was a telegram from Germany's foreign minister to his man in Mexico which offered the Mexicans a return of territories lost to the United States if they joined the war on Germany's side. The telegram was intercepted by British intelligence, and on April 6, 1917, Congress declared war on Germany and her allies.
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Germany started an unrestricted warfare in February 1917. This unrestricted warfare allowed Germany to attack any ships in any water body of British possession. A merchant ship of the United States, known as the Lusitania was sunken as an effect of this, causing the death of many including 129 Americans. This happened on May 7th, 1915. This was one superior reason of the United States' involvement in World War 1.
Another reason was the Zimmerman Note. German foreign secretary Arthur Zimmermann sent a secretive telegram to the German ambassador of the United States to send to Mexico. British spies figured out that in the telegram, Germany was trying to get Mexico to join their side and fight the Allies, and will in return help the Mexicans in regaining some American land that they have lost in the Spanish-American war.
Germany began unrestricted submarine warfare, sinking all ships in a specific area, which caused them to sink ships with American passengers, including the Lusitania.
Another factor is that Arthur Zimmerman of Germany sent Mexico a telegram saying that Germany will help Mexico regain lost territory from the US if they helped Germany win the war. The telegram was intercepted by the US, and Woodrow Wilson declared war on Germany.
In February 1917 the Germans began unrestricted submarine warfare. This meant they were sinking US merchant ships. This led to the United States dissolving all diplomatic relations with Germany. However, President Woodrow Wilson still was trying to keep America out of war and pushing for the country to remain neutral. The tipping point came with the Zimmermann telegram. Arthur Zimmermann, the German Foreign Secretary, issued a telegram to Mexico offering to help them reclaim territories lost to the US if they would launch an attack on the US. The Americans intercepted the telegram and on April 2, 1917 Wilson asked Congress for a declaration of war against the Germans. Congress declared war on April 6.