But they attacked before that. It was an attack that started the war, after all.
A classic modern era attack at that: a military superpower of the time well prepared to crush its smaller and technologically and doctrinally outdated Polish counterpart. Essentially, a zero-risk endeavor, especially when the USSR had agreed not to take advantage and the UK and France could be expected not to react, either. But Poland did defend - and at the borders, too, where defense was guaranteed to fail, rather than at practical fallback positions. For both symbolic reasons and for reasons of alliances, but mainly out of sheer miscalculation.
Quite a good analogy to "Ensigns of Command", too. Hitler's might had been theoretically demonstrated even if not practically proven; the Poles had been sold out in treaties, unknown to them; and it could also be argued that Poland was the creation of hardy pioneers, built out of the ashes of a completely destroyed older realm with the same sort of vigor the colonists had demonstrated in the episode, and that this work had taken place in "good faith" conditions where earlier treaties had indicated Poland could finally be rebuilt after its many forced divisions.
So perhaps not quite so far from where we started after all...