It was a prevailing notion, but it’s been overstated in most teaching of history. A majority could be considered isolationist, or really non-interventionist in both polices that sent arms and money abroad. More ill than harm caused by intervening in world affairs; you were radical for suggesting it. Less in the Northeast than in the rest of the nation.
Consider that while American intervention in ‘The World War’ was short, it stung. More than 50,000 soldiers were lost in combat and another equal amount to disease and wounds accompanying the conflict. All told, 1 out of 300 of the US population was killed or wounded as a result of intervention. Obviously a higher percentage of youth and a much higher ratio of men. And the nation went into recession, labor strife, racial conflict and a horrible flu epidemic after the war. Not as much damage as other powers suffered, but certainly enough to affect attitudes towards fighting a war in Europe again.
The Congress passed several Neutrality Acts in the late 1930’s banning the provision of credit for the purchase of arms (cash purchases were allowed) and allowing the President to ban ships of a belligerent nation in U.S. waters. It also banned Americans from riding belligerent ships (read Spain during the Spanish Civil War) or American merchant ships from carrying arms to warring countries.
So there were these steps taken to insulate us from a devastating war. But we were not a nation of hermits; for instance, Coolidge’s Administration did conclude Kellog-Briand. We led the effort. Officially called the “General Treaty for Renunciation of War as an Instrument of National Policy” treaty, countries promised not to use war to resolve conflict. It did not set up any body to resolve disputes but an isolationist nation would not have signed it. It was signed by Germany, France and the United States on August 27, 1928, and by most other nations soon after. It is named after its authors, United States Secretary of State Frank B. Kellogg and French foreign minister Aristide Briand.
Coolidge also supported the US cooperation with a World Court (a few isolationist Senators filibustered) and Dawes/Harding refinanced the loans to European allies to try to help. The League of Nations proposed by Woodrow Wilson was rejected in the United States, but it was not without its fans, including former President Taft and other GOP members. So America did not fully retreat during the 1920’s and 1930’s; ground troops were a tough sell though.