What lessons can we learn from isolationism in the 1920s?

The best lesson we can learn is that great powers can not be free from foreign commitments, from being involved in global affairs, because this neglect will in the end bring negative consequences. It is imposible, given the degre of interconnection among all nations.

After World War One, president Woodrow Wilson promoted the League of Nations, an international organization that was supposed to promote peace and security in the world. But then the Unites States refused to join it and also refused to ratify the Treaty of Versailles. Isolationism won. Besides, the League of Nations had no legal instruments to enforce their peacekeeping decisions. Lesson were learned the hard way in the 1920s and 1930s, a time that saw the rise of fascim and totalitarianism; another world war and dozens of millions of dead were part of that process. Permanent global involvement of strong nations is inevitable, it's part of the balance and order in the world.