A Whole host of factors prevented an attack in the West, Chamberlain's leadership was only one such factor. As noted above, nether Army was ready for offensive operations until early 1940, by which time numerous German forces had shifted back to the West.
However, if [and a might big one it is] they had somehow managed an attack through Luxembourg and punched into the Rhineland, Germany would have been in a bad way initially, but the Allies lacked the armored formations and doctrine to make a truly decisive blow. Germany's blitzkreig tactics would have been just as difficult to stop when used as a counter attack tool, but the Allies would have had the Rhine as a fallback. Probably a stalemate at the Rhine with Germany industrializing/war focusing earlier but a somewhat more resilient French army facing them.
Some interesting shifts, since the Norwegian campaign probably doesn't occur in this scenario, but the Italians would have been bound by treaty to come in in support of the Reich -- and would do so with a greater proportion of England's air forces (and ground?) committed to the Rhineland campaign.
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