Americans were largely disillusioned after WWI. The US did not join the League of Nations because Congress saw the League as a threat to American sovereignty. The victorious Allies fought among themselves for the colonial spoils of war and the best way to punish Germany. The just peace that Wilson promised never materialized. Many Americans started to view the US's entry into the war as a way to preserve loans made to the Allies. Americans were also shocked by how much American business made off the war compared to the meager pay of the doughboys.
The US was not truly isolationist during the 1920s and 1930s as it intervened heavily in Latin America and also hosted conventions to promote peace such as the Washington Naval Conference. The US did not take on a global leadership role as the American people were tired of the internationalist approach to politics and they found the Republican message of "normalcy" appealing.
The US was also too caught up in its own domestic affairs during the 1930s. The US was in the depths of the Great Depression when Hitler came to power and Japan invaded Manchuria—there was little political interest in going to war against belligerent states. The U.S. did not have a large army, and most of its weapons systems were antiquated by the time the world was getting ready for WWII. Even if there was a political will to stop invasions by Germany and Japan, the US would have had little force to put behind its protests.
Now, the US is not as isolationist as it was during the interwar years. It has taken the lead in the global war on terror and in attempts to curb nuclear proliferation. In the past, the US was willing to act within the framework of the United Nations and NATO in order to accomplish its long-term international goals. However, more recently the US has become more willing to act unilaterally, thus taking on the role of world policeman. This has led to increased military budgets and the alienation of former close allies.