I see no likelihood of these powers not responding.
What happened to Czechoslovakia was just barely justifiable, at least at the start. If the populations of regions populated overwhelmingly by ethnic Germans wanted to leave a young state where they were at an arguable disadvantage, then the secession of the Sudetenland made a certain sense. If this actually would put an end to German irredentism, and so avert war, this might have been defensible.
The subsequent destruction of Czechoslovakia, with the secession of the Slovaks and the imposition of a German protectorate over the Czechs and the annexations of border territories by Poland and Hungary, put an end to this. Germany was not interested in moderately editing the European balance of power, it proved. Rather, it wanted to redraw the map entirely.
Poland would be one step too far. Never mind the relative lack of convincing German claims--the Polish Corridor had a largely Polish population by this point in time, and Germans in Poland were a scattered minority--but Poland was the closest thing to a Great Power in the spaces between Germany and the Soviet Union. Even more to the point, Poland was an ally at least of France. Doing nothing as Germany liquidated the Western powers' only substantial ally in the region was just not an option, especially after the scale of German revisionism had become apparent. They knew perfectly well that they would be next.
I can only imagine Western non-intervention if geopolitics were substantially different, if Britain and France had no stake in central and eastern Europe, or if they were distracted by a conflict elsewhere.