I did some research in response to your comments and the original Walzer article. US neutrality in the Spanish Civil war was inevitable. Of course, FDR was president and enjoyed massive majorities. However, the Democratic party (back then) was the “Catholic” party. Given that American Catholics were very pro-Franco, US neutrality was inevitable. To put this is perspective, the first Republican to get a majority of the Catholic votes was Eisenhower.

Say (very hypothetically) American Protestants made killing Catholic priests a priority. A very violent reaction from American Catholics could be quite reasonably be expected. Fortunately, American history does not include that particular misery. The Ursuline convent was burned (by Protestants) in 1834. No deaths resulted. Much later, JFK won the West Virginia primary in 1960. His victory (de facto) ended the Protestant/Catholic schism in American life.

The US Republicans (back then) had little or no influence (this was the 1930s) and might have supported Franco. However, because of the Depression they were very much out of office.

To put this in perspective. I am not Catholic, but have lived most of my life in very Catholic parts of the US. I tried to marry some number of Catholic girls, they (wisely) turned me down. I have a funny story along these lines. Until I moved to a southern state, a had never met a single US Protestant. This in a country with more Protestants than Catholics