Why did the United States intervene in World War I?

World War I began on July 28, 1914 and lasted until November 11, 1918. Many historians and analysts argue that there are various reasons that led to the start of the First World War. However, the majority agree that it all began when a young Bosnian Serb and member of Young Bosnia (an organization seeking to end the Austro-Hungarian rule over Bosnia and Herzegovina) by the name of Gavrilo Princip assassinated the Austro-Hungarian Archduke Franz Ferdinand in Sarajevo. As a result, Austro-Hungary declared war on Serbia.
During the conflict, Austro-Hungary, Germany, the Ottoman Empire, and Bulgaria (the Central Powers) fought against France, Russia, Great Britain, Italy, Romania, Japan and the United States (the Allied Powers). Because of the new technologies and weapons, the war had a tremendous impact on the world’s social, economic, cultural, and political climate and saw unparalleled levels of chaos and destruction. By the time it was over, nearly 17 million people, both soldiers and civilians, were dead, and countless structures were destroyed and damaged.
In the end, the Allied Powers claimed victory, and the war went down in history as “The Great War.” However, because of the unresolved issues between the participants, a Second World War, much greater and deadlier in proportion, would follow twenty years later.
At the outbreak of the war, the USA decided to remain neutral and ‘observe from the sidelines,’ which was a policy suggested by then-US president Woodrow Wilson. The US government and military continued to collaborate with Europe business- and politics-wise and maintained a stable relationship with both sides, mainly supplying food, money, weapons, and raw material for the Allied Powers.
However, the majority of the population disagreed with President Wilson’s neutrality strategy and believed that the US should enter the war. They became especially enraged when Germany started to attack neutral ships that carried cargo and passengers, declaring the British waters a war zone. This kick-started the campaign known as the U-boat Campaign, in which German U-boats resumed unchecked submarine warfare and attacked several vessels from the Allied Powers and the US, including those carrying passengers.
The turning point happened in May 1915, when German U-boats attacked and sunk the British ocean liner RMS Lusitania - the world’s largest passenger ship at the time. It was traveling from New York to Liverpool, England and had hundreds of American civilians on-board. After this tragic event, the US began preparing for war, with Congress passing a $250 million arms appropriations bill in February 1917.
Meanwhile, German U-boats sunk four more American vessels. Fueled by the American people’s protests, President Woodrow Wilson made an official public appearance, on April 2, calling for a declaration of war against Germany. Thus, on April 6, 1917, Congress voted to declare war on Germany, which marks the official entry of the United States in the First World War.