September 1, 1939 was the beginning of World War II, when Nazi Germany invaded Poland.
The threat of totalitarianism that Nazi Germany represented created a crisis in American foreign policy for President Franklin D. Roosevelt, as isolationists strived to keep America out of World War II as not our war, since we had two great oceans that separated us from war elsewhere in the world.
Many of the isolationists were Republicans, but included naive people from all political persuasions and walks of life.
Americans did not want to get involved in the war, but eventually, it was forced upon us by the Japanese attack on Pearl Harbor on December 7, 1941.
If that had not happened, America might have faced a likely threat from European Fascism without allies.
This is important to keep in mind as the Russia-Ukraine war has ended 18 months and counting, and there is growing demand by isolationists and the right wing of the Republican Party in Congress to stop funding for Ukraine in its defense.
These critics include apologists for Vladimir Putin and the Russian Federation, and they seem unperturbed at the reality that Russia, if successful in Ukraine, will put its sights next on NATO nations, including particularly, Poland, Latvia, Lithuania, and Estonia, but with grand designs to recreate the old Soviet Union before 1989!