The D-Day invasion marked the beginning of the end of World War II. (Photo: REUTERS/Cpt Herman Wall/US National Archives/Handout via Reuters)

Here’s the scenario: A European country is invaded by the powerful military force of a neighbor, and while a lot of Americans are deeply troubled, most people figure it’s not really our fight. Economic issues at home are more important, politicians agree; isolationism, evoked by the slogan “America First,” is alluring.

That’s the way it was 83 years ago, as the last big war in Europe broke out. This week’s invasion of Ukraine by Vladimir Putin’s Russia is different in many ways from what happened in September of 1939, when Adolf Hitler’s troops quickly overran Poland. Yet it’s hard to be too comforted by the contrasts between this 21st-century aggression and the 20th century’s most awful conflict because we’re probably less ready now to face down a substantial threat to freedom than America and its allies were then.

Our weakness isn’t so much in mi…