The primary objective of the United States when World War I broke out was to remain neutral. President Woodrow Wilson worked hard to maintain neutrality, and most Americans initially supported his efforts. This objective, although it succeeded until 1917, was ultimately doomed to failure, due to German attacks on American shipping and the resultant deaths of many Americans.

In the early stages of the war, Germany's intention was to quarantine the British Isles. However, the United States and Britain were strong trading partners. When Germany began to attack all ships that entered the zone around Britain, American lives were lost on sunken foreign ships, and American ships were sunk as well. Initially, Germany apologized for the loss of American lives. However, when it resumed unrestricted attacks on all ships in war zones and several more American ships were sunk, the United States became determined to stop these attacks on their shipping. This US objective in World War I was completely successful, as the Americans helped the Allies win the war and the shipping lanes once again became safe for commerce.

When President Wilson addressed Congress on April 2, 1917 to request a declaration of war, he mentioned another, broader objective: "to bring the government of the German Empire to terms and end the war." In this, of course, the United States was also successful.

However, at the end of his speech, Wilson indicated the ultimate objective of the war: "to bring peace and safety to all nations and make the world itself at last free." In this objective the United States was unsuccessful. After the war, Wilson tried to bring about world peace through the League of Nations, but he failed. Although he won the Nobel Peace Prize for his efforts, the United States never even joined. A few decades later, another, even more devastating world war erupted, and there have been wars in some part of the globe ever since.

The United States had immediate and long-term objectives in entering World War I. The immediate objective was to maintain the right to trade with European powers. This right was under attack (literally) by the German Navy, which, using submarines, attacked American merchant ships bound for Great Britain. Having suspended this practice in 1916, the Germans reinstituted it in 1917, an act which ultimately led the United States into war. American involvement in the war not only secured the ability to trade with the nations of Europe but essentially established the United States as a creditor to Western Europe.

In terms of long-term objectives, Wilson and many like-minded Americans hoped that American entry into the war would establish a post-war order that would avert future conflicts, lead to free trade, and stop what was viewed as a disturbing development in Russia—the Bolshevik Revolution. In this sense, the United States was largely unsuccessful, due in no small part to the isolationist impulse that gripped American politics in the wake of the war. Wilson was unable to implement his vision of a "peace without victory" in the Paris peace talks, and the resulting Treaty of Versailles harshly punished Germany in ways that contributed to the rise of Nazism in that country.

Moreover, the United States refused to ratify the Versailles Treaty out of fear that the League of Nations it established would lead to further American military involvement around the world. Initial American insistence on repayment of wartime loans led to massive inflation in postwar Europe, further upsetting the order Wilson had hoped to establish. Finally, the Soviet Union, despite US actions, developed as a communist power under Lenin and especially Stalin in the postwar years.

So, to the extent that the United States sought to protect its own ability to trade overseas, the war fulfilled its goals. But Wilson was almost completely unable to implement his vision for the world he hoped to create by US entry into the war.

This question is a tough one because a good case can be made for each side.  The opposite truth in each mode of thought is equally valid, making it a challenging question.  In one respect, the United States did achieve its objective in entering World War I.  It sought to stop the Central Forces.  The United States was committed to stopping the Austrian- German alliance and render it useless.  President Wilson was convinced of this threat as hindering democracy.  The belief that Germany was able to facilitate an attack on the United States through Mexico helped to motivate President Wilson in his call to "make the world safe for democracy."  This was interpreted as a call to stop Germany.  The American presence in the war did this, essentially shutting down German advancement.  Implicit in this is the idea that Wilson believed that German submarine warfare endangered the United States.  He was able to parlay this into the idea that Germany posed a threat to democracies and fledgling democracies in Europe.  Wilson's' commitment to stop Germany in World War I represents one aspect in which the United States achieved its objective.

It is here where the opposite truth is equally valid, thus making the question so challenging.  President Wilson believed the war to be a moral one, a war that "would end all wars."  His belief was that the focus of World War I was to end all other types of warfare with a fervor that underscored American commitment in it.  This objective of the war as a "moral crusade" failed.  The peace talks that led to the drafting of the Treaty of Versailles reflected this failure.  President Wilson's commitment to see World War I as the "war to end all wars" was abruptly truncated when the presence of secret alliances still existed in the "peace" talks.  Open acceptance of German reparations and a punitive judgment upon Germany essentially nullified Wilson's hope that World War I would be the final example of world conflict.  The belief of "making the world safe for democracy" was undercut with a treaty that acted more like an armistice.  Democracy was no more safe with the ending of the war than it was before it.  The rise of the charismatic dictator in Italy, the Soviet Union, Spain, and Germany became the direct result that followed the end of World War I.  Democracy was more under siege in this period than in the period that preceded World War I.  It is here in which a very plausible case can be made that the United States failed to achieve its objective of idealism in the conclusion of World War I.