get slovakia to invade first over some border issue as a pretext then have the ussr invade the next day then invade later that same day or earlier the next day after sending a flurry of vague ultimatums and demands to the poles with the free city of danzig voting to reunite with the reich on that same day and announcing that poland is invading it and calling upon the reich for assistance. in this way it'd look to a sympathetic observer that the poles bit off more than they could chew, starting a fight with slovakia, getting invaded by the ussr and then, in hopes of securing a victory in their desperate situation, trying to annex danzig thinking the germans would back down. just timing things right would've provided more than enough of a fig leaf for the allies to not intervene.
United Kingdom