You wrote:
“Sorry. Everyone knew in 1939 that Jews were in danger in Germany. The Jews on the St. Louis didn’t go back to Germany.”
Sorry, but the Jews on the St. Louis were FROM GERMANY and if not for the active intervention of the Brits, French, et al. Germany is exactly where they would have ended up. Did that not occur to you? Perhaps you didn’t know that people are deported to their nation of origin or that ships are ultimately sent back to their nations of origin? The St. Louis was a German ship, registered in Hamburg, Germany. Did you know that?
“In 1939, France had the largest Army in Europe.”
Irrelevant.
“It wasn’t obvious to anybody in 1939 that France would be conquered by Germany in a month, nor that Germany would seek to exterminate the Jews when in 1939 it was simply trying to expel them.”
Again, irrelevant. Kristallnacht had already happened. It already was well known that Jews suffered horribly in Germany and that Jews could lose their lives in Germany for no other reason than that they were Jewish.
“In January 1942, the Nazis stopped trying to expel the Jews from their territory and embarked on a radically new approach — extermination. That happened 3 years AFTER the St. Louis incident.”
And Kristallnacht happened one year BEFORE the St. Louis incident. Keep up.
But what's your point? There was never any realistic threat of the St. Louis passengers being sent back to Germany. And it's not like the US didn't accept a large number of Jewish refugees in the 1930s. The US did accept a large number of Jews, even though jobs were scarce in the Great Depression and immigration was incredibly unpopular.
. Kristallnacht had already happened.
Which indicated beyond any shadow of a doubt that it was unsafe for Jews to stay in Germany. Every German Jew who had any desire to leave Germany was able to get out, either to Britain, America, or, for the less lucky, France, Poland, Denmark and the low countries. 60 years later we know that Belgium, Poland and France weren't safe, but nobody knew that in 1939.
“In 1939, France had the largest Army in Europe.” Irrelevant.
No, not irrelevant. From the perspective of any observer in 1939, French Jews were safe.