Why did the United States side with Britain during World War l?

This is a question with a complex answer, as it involves a lot of factors.

Both the Central Powers (like Germany) and the Allied Powers (like Britain) had made propaganda to try and convince the United States to join their side. However, President Wilson pursued a policy of neutrality and kept the United States out of the conflict for a while.

Because of this neutrality policy, the United States shipped goods to both Germany and Britain. However, as the war went on, Britain began seizing US goods headed for Germany. In response to this, Germany began increasing submarine warfare in the waters around Britain and in the North Atlantic. The targets of these submarines (or U-boats) were merchant ships at first.

However, in May 1915, a German U-boat sunk the Lusitania , a British passenger ship, resulting in over 1,200 deaths (including 128 Americans). Many people in the US reacted with outrage at this seemingly unprovoked attack (the Germans believed that the ship had been carrying munitions for the war). While Germany pledged not to sink any further unarmed ships, this pledge did not last.

Due to increasing pressure to break the stalemate on the Western Front of the war, German U-boats eventually began targeting unarmed ships again in late 1916 and early 1917. President Wilson broke off diplomatic relations with Germany at this point, but was hesitant to call for a declaration of war.

Two major series of events changed this in the spring of 1917. First, four German attacks in the span of two months on unarmed US ships caused major outrage. Additionally, the revelation of the Zimmerman telegram occurred. The telegram (from German Foreign Secretary Arthur Zimmerman) had been sent to Mexico, asking for Mexico's support in the war in exchange for the return of southwestern US territories like Texas, New Mexico, and Arizona.

Unable to avoid getting involved any longer, President Wilson asked for a declaration of war on April 2, 1917, and the US went to war against Germany - that is, on the side of Britain.