1939, June to September: Summer crisis
The Polish government does not want to cede Polish soil to Germany. British prime minister Chamberlain calls for a conference. Participants are: Great Britain, Germany, France, Italy and Poland. This is a great difference to the Munich conference of last year's autumn.
The results are:
- German annexation of Danzig
- Germany receives an exterritorial way from Pommerania to Danzig
- Germany receives part of upper Silesia.
- Polish traders in Danzig will be treated as German traders
- Germany accepts all borders in Europe as unchangeable
Polish government wouldn't cede any territory to Germany, period. Officials who'd've tried it, would be ousted.
1939, November
Stalin invades Finland. He did it because he thought the economical problems Germany is facing (Mefo-Wechsel a.s.o.) will stop it from intervening. He is right.
But it wouldn't stop UK&France from intervening

And in this TL there's no hostile Germany to prevent movement of troops through Baltic Sea...
1940, Spring
Poland, Lithuania, Latvia and Estonia sign a military alliance. France and Great Britain guarantee the borders.
Doubtfull. Relations between Poland and Lithuania were really bad. Also, Baltic states'd worry about becoming a target for USSR - so no. Well, no announced to the all of world agreement. The secret defence pact - maybe...
1940, July
Soviet troups beat the sh** out of Polish cavalary and reach Warsaw and the Weichsel river in early August.
...what's with people assuming that every war in which Poland faces against Germany or Russia will unfold like OTL September 1939? Especially attack from the east...
Lets see what's different:
1. Geography
Length of Polish borders in 1939:
with Germany - around 2500 km
with USSR - 1412 km
Also, look at the shape of the borders - the shape of German one makes Polish forces outflanked from the day 1 of war, while the border with USSR is, from a strategic viewpoint, a straight line.
Another thing - the Prypec swamps. In the middle of border lies a stretch of swamps impossible to cross over by large fighting force, effectively dividing the theatre of war into northern (Belarusian) and southern (Ukrainian) halves. Meaning it divides any attack from east into two, unable to support each other, thrusts.
2. Transport, economy, supplies, population.
Road density was much lower in eastern Poland than in western, so the speed of advance would be much slower.
Most of factories and supply depots lied in western Poland, far from possible frontline with Red Army - unlike the situation with Germans. So in war with USSR Polish army would be much better supplied than in OTL September 1939.
Moreover most populous regions of Poland were the western ones.
3. Fighting force
While USSR would send against Poland more troops, airplanes etc. than Germany, they'd be, compared to Wehrmacht/Luftwaffe/Kriegsmarine:
a) much worse equipped
b) much worse commanded
c) much worse trained
d) with much worse logistical chain
etc.
And in abovementioned categories, Polish Army would have advantage too - unlike against Germany, where high command and logistics were really awful (training and equipment levels were similiar).
And on the side note: just because "Soviet
troops beat the sh** out of Polish
cavalry" (all of 9 brigades of it) does mean nothing, as there's at least 39 infantry divisions and one motorised brigades left... oh, wait, it's mid-1940. So it'd be 8 cavalry brigades and 2 motorised ones.
That's it for now.