It is at least possible to argue that these two countries were partially responsible for Hitler’s rise to power. A full accounting for that rise would have to hold the people of Germany responsible. It would also hold the French and British more responsible than the Americans.
We must, of course, realize that it was the German people who allowed Hitler and the Nazis to rise. They were not forced to do so. The ultimate responsibility is theirs.
That said, the English and the French are partly responsible for the rise of Hitler. This is because these two countries insisted upon punishing Germany harshly in the Treaty of Versailles that put an end to World War I. President Wilson of the United States had not wanted to do this. French and British demands for reparations and other punishments helped bring about the economic and political turmoil that helped bring Hitler to power.
The British, French, and Americans can all be blamed to some extent for Hitler’s rise since they were all involved to some degree in appeasing him after he became Chancellor. If any of these countries had stood up to Hitler when he, for example, remilitarized the Rhineland, he might well have been stopped from building Germany into the power it became. France and Britain bear more responsibility for this than the US because they were in a much better geographical position to push back against Hitler.
Thus, the US bears some responsibility while the British and French bear more. The ultimate responsibility, though, lies on the Germans.