If Britain and France had shown some fortitude in 1938 and had told Hitler to pound sand there is a very good chance September 1, 1939 would not have turned out the way it did.
Even several of Germany’s generals said that if there had been pushback against Germany’s remilitarization of the Rhineland in 1936, and a strong opposition to Germany in the Czech Crisis of 1938, Germany would have backed off.
Hindsight is always 20/20, the reason they didn’t was because the war 20 years earlier had bled France and England white. Nobody wanted a replay of that.
Keep in mind, over 19,000 British troops were killed in one day, first battle of the Somme, maybe 50,000 wounded. They’d seen this movie before, so to speak.