The war that consumed the Great Powers of Europe was not America's war. In fact, at the beginning of the conflict, most Americans were happy to allow the Europeans to destroy themselves rather than squandering American lives and money by intervening in Europe's Great War. Over time, however, this sentiment—known as isolationism—began to waver as the war dragged on and ideological concerns joined with more mundane issues like freedom of the seas to provoke President Woodrow Wilson and Congress to send American soldiers to the trenches of Europe.
Possibly the most surprising thing about America's involvement in the Great War (great as in big, not great as in really cool) was that it happened at all. President Wilson ran for reelection in 1916 and won by embracing the slogan "He kept us out of war." Much of Wilson's platform had to do with following George Washington's suggestion in his Farewell Address to avoid foreign entanglements, in this case keeping America out of the war in Europe. Added to that was a strong pro-German sentiment in many parts of the country, most notably among the German immigrants and their descendents in the Midwest, which made the choice of sides somewhat unclear.
Unlike during World War II when American opinion was far more anti-German, World War I was less about ideology and good vs. evil, and therefore both sides in the conflict gained support from the American people. Part of the problem for those advocating intervention, however, was that no one really knew what the war was about. In August 1914, it was a war over tensions between Serbia and Austria-Hungary in the Balkan Peninsula; but by 1917—when the United States entered the war on the side of Britain, France and (briefly) Russia—the goal of the war had become simply to win. And winning in this war of attrition meant killing more of the enemy's soldiers than he killed of yours—not a situation in which an American president wanted to get involved.
At the beginning of the twentieth century, the United States was not the world power it would be fifty years later. America's last war of aggression— Spanish-American War—began a slow change in America's role in the world. For the first time, the United States owned foreign territories, and for the first time, Americans could rank themselves among the imperial nations of the day. By 1914, moreover, the U.S. had almost no Army, and its Navy was not up to par with those of the Britain and Germany. Isolationism (at least, when it came to European entanglements) was not just a fad, it was a way of life, and most Americans did not want to intervene in the war in Europe. And by the time public opinion changed, it was almost too late.
The principal reasons America entered the war were submarines and propaganda. German submarines, called U-Boats, were a new and terrifying weapon in 1914. They were also seen as unchivalrous: they hid in the ocean and attacked without giving their enemies any warning, killing innocent civilians and military personnel at the same time. They were also very effective at destroying Allied shipping in the Atlantic. The most famous U-Boat sinking was the passenger liner Lusitania, sunk on 7 May 1915 off the Irish coast. Controversy erupted immediately on both sides of the Atlantic. The Germans claimed that the Lusitania was carrying war materiel, citing the fact that only one torpedo was fired, yet there were several large explosions. However, with 128 Americans dead and public opinion appalled by this "outrage" against civilized society, the United States demanded and received an informal assurance from Germany that it would stop unrestricted submarine warfare against neutral shipping. In English, that meant that the U-Boats would no longer shoot any ship entering into the zone of their attempted blockade of Britain but would instead make positive identification that the ships they were attacking were military ships.
With U-Boats mostly out of the equation, the United States entered into an interesting limbo. While technically neutral and taking no side in the war, the U.S. was the only major industrial power from which the Europeans could buy weapons and food. And while the U.S. was willing to send arms to anyone who purchased them, the reality of a British blockade of German and Austrian ports meant that only France and Britain were actually buying American products. Lots and lots of products: ships, foodstuffs, guns, bullets, uniforms and everything else you can think of. This had the effect of tying the United States closer and closer to the allies, and since the U-Boats could not cut the lifeline of Atlantic trade due to the restrictions the Americans put on their use, Britain, France and the United States slowly moved onto the same side.
Helping move the United States to support the Allies was a massive British propaganda operation, the likes of which the world had never seen before. From the very beginning of the war and the so-called "Rape of Belgium" in which the German Army was portrayed as defiling innocent Belgians (and even of raping nuns), British propaganda efforts had worked hard to keep up morale at home and to gather more allies for the Allies.
This effort came to a head with the Zimmermann Telegram in January 1917. British intelligence intercepted a telegram from German Foreign Secretary Arthur Zimmermann, to the German ambassador in Mexico, Heinrich von Eckardt, telling von Eckhardt to try to get Mexico to fight against the United States alongside Germany. The Germans promised huge financial and military support to Mexico, as well as assuring the Mexicans that when the war was won, Mexico could recover its "Lost Territories" of the American Southwest. Mexico declined the offer but not before the British leaked the telegram. It had an unbelievable effect on American public opinion, and anti-German sentiment went through the roof.
Then, on 1 February 1917, Germany informed all nations that it would resume unrestricted submarine warfare against all ships from all nations in European waters. Without the submarine menace, Britain and France were receiving huge quantities of war supplies from the United States, so this declaration had the effect of making it almost inevitable that America would enter the war. On 2 April 1917, Wilson went before Congress and asked for a declaration of war on the basis of "making the world safe for democracy." On 6 April Congress voted America into World War I, with six Senators and fifty Congressmen voting against involvement in the conflict. America was at war.
By and large, Americans rallied around their president and Congress and supported the war effort. Men enlisted by the thousands, and Wilson rammed through a Selective Service Act to institute a national draft. Anti-war sentiment still existed in some circles, but overall those who avoided the war were known as "shirkers" and were not treated kindly.
Though the total number of objectors is hard to know, it is illustrative that the government was worried enough about antiwar sentiment that Congress passed several repressive, anti-opposition laws were passed and vigorously upheld by the government. The Espionage Act of 1917 and the Sedition Act of 1918 were aimed at repressing dissent during wartime. The effect of these acts was to throw into jail prominent members of the Socialist Party, including presidential candidate Eugene V. Debs, and anyone else who spoke out against the war. The irony of such laws to prevent citizens from speaking out against a war ostensibly being fought for a democratic world seemed to be lost on Wilson and Congress.
To enlist enough troops, Wilson forced through the Selective Service Act of 1917, organizing the first national draft in American history. Though not overly popular, the measure allowed the U.S. to raise over three million soldiers in a matter of months, something that voluntary enlistments could never have accomplished.
War bonds were sold to raise money and women made bandages much as they had during the Civil War. But this time women were also asked to work in factories and take over other jobs that were traditionally done by men. Unlike their daughters twenty-five years later, these women did not demand to stay in their factory jobs after the war was over, but it was a major step forward for women's rights in America. In fact, when President Wilson finally came out in support of the Nineteenth Amendment, granting women's suffrage, he did so by arguing that it was an urgently needed war measure that would help bring the nation together during the difficulties of wartime.
America's European Allies had their own ideas about how the American soldiers could best be used in battle. Suffering unbelievable casualty rates and exhausted by two and half years of gruesome warfare, the French and British high command saw the American troops as a perfect stop-gap measure. They instructed General John J. "Black Jack" Pershing, commander of the American Expeditionary Force (AEF), to put small American units into the line under the control of French and British commanders. Pershing would have none of it, instead negotiating, obfuscating, and delaying until larger American units were trained and could be put into the trench line under their own commanders. To the dismay of the Europeans, this meant that there were still few American troops on the line when the Germans launched their Spring Offensive in 1918, a last-ditch effort to break the Allied line, which nearly succeeded and was finally stopped in part by brave action of the U.S. Marine Corps.
Throughout the war the American command had to deal with their European counterparts in a partnership that had its ups and downs. This was best illustrated after the Armistice was signed on 11 November 1918 when Wilson became the first president to travel abroad while in office, so that he could personally appear in Paris to negotiate the Treaty of Versailles. There, Wilson elaborated his Fourteen Points, a plan for reordering the world according to his idealistic notions of peace, justice and democracy. They were a huge failure because Britain and France were not interested in being nice to Germany after such a brutal war, and the one thing that Wilson did get, a League of Nations, was defeated in the American Senate, which had returned to its isolationist roots following the war. American public opinion had clearly changed after the war: Wilson's Democratic Party lost majorities in Congress and the presidency by 1920.
Despite Wilson's failure at Versailles, World War I was the first time that the United States had flexed its military muscle far from its own shores against the so-called Great Powers of Europe. With America providing both the economic and military power to finally win the war, the U.S. solidified its position as a nation to be reckoned with on the international stage.