Here is my take:
- Stalin threatens to break non-aggression pact USSR has with Poland (hoping they'll get scared and fold) if they participate in partition of Czechoslovakia. When they do, he breaks the pact, not wanting to look weak, abroad and in home.
- Poland feels endangered, and agrees to join anticomintern pact. Details of the agreement might vary, but its more-or-less fair deal.
- Germans break Munich agreement, and annex remnants of Czechoslovakia.
- in 1940, Germany surprise-attacks France and Low Countries, UK downs them in retalation. (no phony war, means less time for France and UK to prepare)
- no formal war between western allies and Poland, but polish trade ships often get sunk/captured by Royal Navy
- France falls about the same way as in OTL, perhaps faster, since Germans did not waste ammunition and equipment fighting Poland. Lets assume Armstice terms are same as in OTL.
- Stalin is horrified how quickly France fell, and prepares his defences (as in OTL).
- Yugoslavia remains in pact of Steel (no coup against prince-regent) because relations are better (seemingly friendly Polish-German means Yugoslavia, which was friendly with Poland considers Germany more trustworthy than IOTL), so Germans do not waste time invading it (even if they still had to fight Greece. Gee, thanks Benito.)
- After failed battle for Britain, join German-Polish army attacks USSR.
- There is shorter distance from eastern 1939 Polish border to Moscow, than from 1941 German-USSR border. German surprise bombing of Soviet airfield has further reach, deeper inside their territory - more planes than IOTL gets destroyed. Also, Polish army that was geared specifically in fight against Soviets (because Poles in early 1930s expected Germany to be bound by Versailles Treaty, and unable to rearm) participates on Axis side, rather than resisting it in 1939. This is slightly offset by Finland remaining neutral.
- Moscow falls before winter. Counterrevolution on the streets, those party commissars that failed to escape by last trains going east get lynched by mob - rotten structure collapses.
- Japan joins in, delaying Pearl Harbor until early 1942. US entry in the war does not save whats left of USSR (Hitler dows US as in OTL).
- Emerging Siberian warlords get subjugated by Germany and Japan, but they keep some autonomy - Germany wants to "take care of" area west of Urals first, while propping up puppet states is Japanese modus-operandi. Poland gets its (small) slice of Ukraine and Belarus.
- Germany, Italy, and Japan then fight Western Allies. Poland supplies Germany with resources, money, supplies, and "civilian volunteers", but does not officially join war.
- Occupation of USSR (Drang nach Osten) drains German resources and manpower, while they are losing ground in Africa and Pacific.
- Italy and Normandy get invaded. Italy switches sides (fails as IRL, and gets occupied). Paris liberated
- Poland switches sides, when half of Germany is under American occupation, and stabs Germany in the back. German units in Russia cut from support.
- in 1945 US drops atom bomb on Hamburg/Dresden/Kiel/Berlin (take your pick...)
- Hitler kills himself. Germany surrenders, Poland takes over eastern Germany (not all it did IRL, since they hadn't lost territory to USSR).
- Non-communist Russian state is created. Russia is spared another 50 years of communism. Vladimir Romanov and Alexander Kerensky arrive in *Jalta Peace conference, and try to influnce future of Russia (borders, form of government, etc).
- Atom bombs are dropped on Japan. It surrenders.
- No Soviet occupation of Manchuria = no Mao takeover of this area. China is spared Communism, Great Leap Forward, and Cultural Revolution. It just has to put up with mundane corruption of KMT.
- Baltic states remain independent. Eastern Europe does not fall to communism.
The only real losers, are those citizens of USSR that nazi would kill in 1942-1945 occupation. But again, millions of Russians soldiers who died in battle, or civilians lost collateral damage of war survived. I don't know what net result would be, so I don't claim it would be more or less dead.
Oh, and those nuked Germans. Suck to be them.
Poland probably wouldn't be able to hold on all their gains in East, but they could on some. At worst they'll keep 1939 borders in the long run.
It rest on two assumptions:
- USSR would do something that would push Poland into German hands - Stalin does not need to do it on purpose, just genuine mistake (he did his own share of those...), for example Soviet spy ring being busted trying to murder prominent person. It could happen, it just didn't. No need for ASB.
- Polish Army fighting for Germany, and not against it, would be enough to turn the tide in the East. I repeat, 1000000 men, equipped specifically to fight Soviets. Again, no need for ASB.
What if Poland join Barbarrossa, but Soviets still win?
Even if Soviets wouldn't fallen, its still better for Poles.
Germans don't murder Poles, or Polish Jews, like they didn't murder Hungarian or Italian ones (until those countries tried and failed to switch sides). No mass graves in Poland until 1944, when Soviets enter.
Soviets weren't harsher on Poles in TTL, since in OTL they were brutal enough to nominally allied Polish population. Good luck trying to outdo themselves.
Poland still gets Eastern German provinces, because:
- Stalin wanted to weaken Germany, so it wouldn't be future threat.
- Stalin didn't knew if he'll get vassal Eastern Germany, or if it'll be unified and pro-west. So pushing borders of Communist Poland that already was in his pocket, was sureway to his area of influence will be larger.
- the idea that Stalin gave Poland eastern Germany because he was so grateful that they fought Germans, so he shed tears of joy over heroism of Polish people, is frankly absurd.