The United States entered the war because of the Germans' decision to resume the policy of unrestricted submarine warfare, and the so-called "Zimmerman telegram," intercepted by the British, in which Germany floated the idea of an alliance with Mexico. Unrestricted submarine warfare, a desperate effort to counter the British blockade of Germany, would lead to the sinking of American merchant ships heading for England, and had been employed by the Germans before. They abandoned it in the face of US pressure earlier in the war. Its resumption was enough to cause Woodrow Wilson to renounce his stated position of neutrality, as his war speech to Congress demonstrates:

The new policy has swept every restriction aside. Vessels of every kind, whatever their flag, their character, their cargo, their destination, their errand, have been ruthlessly sent to the bottom without warning and without thought of help or mercy for those on board, the vessels of friendly neutrals along with those of belligerents.

While there was significant opposition to the war in the United States, the official position was that the nation could not tolerate such an imposition on its rights as a sovereign nation, to say nothing of the effect of the Zimmerman telegram. American entry into the war broke what had been a bloody stalemate. US troops were instrumental in repulsing a German offensive, and led the way in an Allied offensive, the combined effect of which fatally weakened the German army. Politically, many Europeans hoped that US entry in the war would result in a treaty based on Wilson's Fourteen Points, but the Treaty of Versailles that officially ended the conflict contained few of its provisions.

World War I is the "Great War", the war to end all wars. The United States wanted the world to know, we were NEUTRAL, but were we really?

Politically, economically and psychologically, we were not isolated by any means. The U.S. did not want to commit to a war, continents away. Let those in war's path do the dirty work, we would make money off of their involvment. It was enevitable and we were doing our part in other ways.

We sold our allies war supplies, remember the Lusitania and so many ships traveling cross-Atlantic?

We made gained ecomonic profits, we did see our young men rally, volunteer and die with our allies, and yes we provided strategic support in many realms.

Did we have a choice? Neutrality in its pure form was not an option. The Industrial Revolution bound the world together. We could not publically commit in 1914, but in 1917 we had little choice due to the losses of our allies.

The United States people saw itself as the "savior" of the righteous, but our politicians had already set us up by involving the U.S. behind closed doors.

This was a war that resulted from Industrialization, Nationalism and Imperialism. The world was changed forever. Larger countries felt a moral duty to defend and profit.

Consider this, the world was in a great depression and the world was left in a greater depression following WWI.

One thing we would never stand for was the "Zimmerman" telegraph event. The Monroe Doctorine would be defended, no matter what the cost.

While the official trigger was the resumption of unrestricted submarine warfare and of course the emotional reaction to the Zimmerman note, I think more crucial was the combination of the ideological sympathy of the progressive with the Allied Powers over the Central Powers, and the strong financial incentive for American Capital for an Allied victory, of which they were heavily invested.  President Wilson also believed by entering the war on the side of the allies he could affect the terms of the peace to provide a just and lasting piece, something he did not think he could influence as a neutral observer.  While there was a sizeable population of Americans of german descent in the United States as well as those of Irish descent who were very unsympathetic with the United Kingdom, the overwhelming economic ties in Europe were to the Western Allies over the Central Powers.  Thus the British Blockade of the Central Powers did not cause the same degree of reaction as the German blockade of the U.K. and France.  Submarines were also viewed as underhanded and "sneaky" as opposed to the surface vessels used by the British in their blockade.

The United States, led by President Woodrow Wilson, did not want to be involved in World War I. When Wilson ran for re-election in 1916, his campaign slogan was "He kept us out of the war" referring to the European conflict that was then referred to as the "Great War." However, by 1917, sentiment was changing with regard to the war.

Although the U.S. had yet to become officially involved in the war, most Americans supported the Allies and saw the Germans as aggressors. Americans felt a connection to Great Britain, in particular, because of cultural similarities. Germany angered Americans with their U-boats sinking American ships suspected of aiding the Allies. With the sinking of the Lusitania in 1915, a British cruise ship, 128 Americans were among those who lost their lives. The Germans temporarily halted such actions but in 1917 resumed unrestricted Naval warfare. At the same time, the British intercepted a message, called the Zimmerman Note, asking the government of Mexico to declare war on the U.S., if war broke out between the U. S. and Germany. The note also promised to help Mexico regain the territory of Texas, Arizona, and New Mexico if Mexico declared war on the United States. This note was the final push that Wilson needed to turn public sentiment towards war. The U.S. Congress declared war on Germany on April 6, 1917.

Since the beginning of World War I in 1914, the United States, under President Woodrow Wilson, had maintained strict neutrality, other than providing material assistance to the Allies. Even in May 1915, when a German submarine sank the British ocean liner Lusitania, killing 128 U.S. citizens out of a total 1,200 dead, the United States, though in uproar, remained neutral.

In January 1917, Germany announced that it would lift all restrictions on submarine warfare starting on February 1. This declaration meant that German U-boat commanders were suddenly authorized to sink all ships that they believed to be providing aid of any sort to the Allies. Because the primary goal was to starve Britain into surrendering, the German effort would focus largely on ships crossing the Atlantic from the United States and Canada.

The first victim of this new policy was the American cargo ship Housatonic, which a German U-boat sank on February 3, 1917. Although Wilson tried hard to keep the United States neutral, by the spring of 1917, the situation had changed significantly, and neutrality no longer seemed feasible. Germany’s unrestricted submarine warfare was taking its toll, as American ships, both cargo and passenger, were sunk one after another. Finally, on April 2, Wilson appeared before Congress and requested a declaration of war. Congress responded within days, officially declaring war on Germany on April 6, 1917.