In a since-deleted writeup on this subject, jbird said, "Ratified on June 28th, 1919, the Treaty of Versailles was a peace agreement between Germany and the Western allies (England, France, America and Italy)." If US President Woodrow Wilson had had his way, that statement would have been true. But the United States of America never ratified the Treaty of Versailles.
This makes little sense; as the deciding player in the war and the dominant power in the world, the US should have been and indeed was the leader in the treaty negotiations. Wilson, however, chose to negotiate more or less alone, along with only a few fellow Democrats. The Republicans in Congress, who had loyally supported Wilson during the war, were in fact spurned twice, as Wilson made a call for all Americans to vote Democratic to "show their patriotism."
This high-and-mighty attitude might have been a footnote to history had the Republicans not won a landslide victory in the midterm election of 1918 and gained control of the Senate. Those opposed to the treaty now had a group of disgruntled, betrayed Republicans to support them. The Senate was essentially divided in three for the treaty debate. There were the Democrats, who mostly supported Wilson and the treaty. A moderate group was led by Henry Cabot Lodge; they would accept a modified treaty and made up the majority of the Senate. Then there were the "irreconcilables," a group opposed to the basic grounds of the treaty which was led by William Borah, Hiram Johnson, and Robert La Follete.
The irreconcilables had various reasons to oppose the Treaty of Versailles, but the most notable are the major problems mentioned in the above writeups (most of which have since been deleted—Oh well, that's life. –Ed.). Their position can be seen as somewhat prophetic in that they believed that harsh sanctions would only lead to more unrest. Lodge and the moderates, however, only disliked one facet of the treaty: Article X, the clause that created the League of Nations.
The horrors of the first World War had created an extremely isolationist feeling in most of America; the average person felt that we should leave the Old World and its problems alone and thus opposed the League. Lodge built on this and also on the fear that the League of Nations would have the ability to supersede the US on military matters and therefore violated the US Constitution. When Wilson refused to compromise, clutching to his precious League, Lodge threw his support to the irreconcilables. In a last-ditch effort to push the treaty through, Wilson went on a nationwide stumping campaign. He was stopped in his tracks by a stroke on September 25, 1919, and the treaty was killed.
As a consequence of the treaty fight, the United States never joined the League of Nations, which is perhaps the greatest failure and embarrassment of Wilson's presidency. The failure of the League had dire consequences for the world, because there was no one to curb the rise of fascism in the 1930's. It also led the US to formally end the war with Germany separately, through the Treaty of Berlin.
Sources: (in addition to my own memory)
Purvis, Thomas L. A Dictionary of American History. 1997, Blackwell Publishers, Inc.
Faragher, John Mack, et al. Out of Many: A History of the American People. 2000, Prentice-Hall, Inc.