Since the pact [Józef] Beck refused to join provided solely for opposition to the international organization of Communism, and [Warsaw] was vehemently anti‐Communist as well as on the least friendly terms possible with the Soviet Union, why the reluctance of [Warsaw] to join and why the insistence of [Berlin] that she do so?

The answer to the question of [Warsaw’s] attitude is that adherence to the Anti‐Comintern Pact was seen as a form of policy subservience to Germany, a subservience not only in regard to the Soviet Union but in every other respect as well. This was no idle speculation on Beck’s part. Once [Warsaw] took the step [Berlin] wanted, she would be at [Berlin’s] mercy because she would have ruptured her tie to France while exposing herself to the wrath of her great eastern neighbor.

It was, of course, this very subservience that Hitler wanted. For his ambitions further east he needed either Poland’s acquiescence or that country’s destruction; any truly independent Poland would be a bar to his aims, and in the immediate future he especially wanted Poland quiet while he settled with England and France.

(Source.)