Poland wasn't the first stop in that line, though, only the one that started the thing. The annexation of the Sudetenland was the first "big thing" after a series of smaller appeasements. The powers of the time recognized that Germany was absolitely unreliable. If the Nazis had framed the Polish campaign as "liberation" following a short-lived Soviet occupation... that might have worked. And may have even resulted in a Nazi/British/French alliance. It'd then come down to how much the Allies' intelligence agencies knew about Nazi/Soviet collaboration on Poland.