Why did the Republican Isolationists refuse to ratify President Wilson's League of Nations?
They viewed it as an intolerabe step toward the end of American sovereignty.
The Isolationist tradition of the American Coservative movement can be summed up by the formula "America First"(the same than Lindbergh's)
Just like Robert Taft who opposed both NATO and the United Nations, they opposed international organizations. Isolationists believed that not only should the USA not intervene in foreigh wars but they also thought that no supranational entity should exist.
This caused the creation of the League of Nations by Wilson, but the US was not in it. Later, faulty constitution and rules caused the League to be pretty much powerless, and every decision took years to make. It was eventually disbanded.
It violated the constitution (Congress could no longer have a say in wars they did not declare).
Shortly after WWI, President Wilson proposed the Fourteen Points, which was a plan created to prevent further conflict and make up for the devastation of the first world war.
Of these Fourteen Points, one point included the League of Nations. This was the first international peace keeping group that would be made, but it failed. For many reasons, the League of Nations was not ratified.
The main reason was because people thought that the League would bring the U.S. into wars without their say. This violates the constitution in that only Congress has the power to declare war. If the League ever saw a country in need of support, they would likely drag the U.S. into the conflict without Congress approval, or any approval at all.
Because of this, the U.S. was ironically not part of the League of Nations that it's president proposed. Not only this, but many key powers in Europe weren't a part of the League.
Since the U.S. and some other powers were not a part of the League; it failed to sustain itself.