The problem is, why would Britain attack?
At that time, the RAF was still non-existant as a useful force, and there wasnt a usable radar chain to direct it. As everyone was convinced that bombing would destroy nations, its rather easier ro see why the British wouldnt intervene (Chamberlains actions were still stupid, but I suspect noone would argue too much at the time)
The French were also terrfied of the Luftwaffe at the time, so again would they have done anything?
It basically falls to the French anyway, if they do nothing Britain cant do anything meaningful.
Even if France and Britain declare war, can the French get themselve in motion before Germany effectively wins? They werent exactlly paragons of mobility in 1939...
And while Poland would be a big help, again they arent going to do anything unless France and Britain declare war and look like doing something effective - they'd mobilise, I suspect, but not go any further until they could see what was hapenning.
I'm also wary of the stock 'oh, the Generals would have deposed Hitler' that keeps getting tossed around in these scenarios. Even after years of dire descisions, they didnt exactly do a sterling job of getting rid of him, did they....?