President Woodrow Wilson made several errors that doomed the Treaty of Versailles and the League of Nations covenant. These were the two primary agreements made at the end of World War I (1914–1918).

First, he made some bad decisions at the end of the war. He alienated Republicans by asking voters to support only Democrats in the 1918 election. Second, he spent a long six months in Paris at the peace conference, and he lost touch with developments back home. Also, he did not include any Republicans on his peace delegation.

In Paris, Wilson placed too much emphasis on the importance of the League of Nations. He had formidable opponents back in the United States. Former president Teddy Roosevelt opposed him. Henry Cabot Lodge, Roosevelt's friend, was chairman of the Senate Foreign Relations Committee. Lodge and others argued that the League could drag the country into a war and that only the Senate had the power to declare war.

When Wilson returned home, he was needlessly confrontational with Republicans. His personal enmity with Lodge doomed any chance of a compromise. There were sixteen "irreconcilables"—mostly Republicans—in the Senate, who opposed Wilson. Trying to go around the Senate, the president embarked on a speaking tour of the nation. His health failed and he collapsed. He never fully recovered, and his impaired health made him even more reluctant to compromise. The United States never did sign the Versailles Treaty or join the League of Nations. Wilson's grand plans for a new, post-war world order ended in abject failure.

I assume that you are asking about Wilson's refusal to compromise with such opponents of the League of Nations as Henry Cabot Lodge.  If so, Wilson's refusal doomed the treaty in the US Senate because it caused Senators to have to choose between Wilson's way or nothing.  Many Senators would have been willing to vote for the treaty if it had had a few modifications.  But Wilson rejected any changes, insisting that the treaty had to passed exactly as he submitted it.  This led to a situation where he encouraged Senators to vote against the treaty because his opponents had attached conditions to it.