For Soviet Union in August 1939 there was risky situation of possible international isolation and aggression of Germany together with its allies.
How could be the USSR invaded in August 1939 if there was no German-Soviet border?
Romania, Bulgaria and Hungary were not German allies at that time.
Romania had an Anti-Soviet alliance signed with Poland in case of Soviet invasion against any of these countries.
Of course relations between Poland and Romania were warm also regarding their German policy, but Romania was not obliged to actively help Poland in case of German invasion, while in case of Soviet invasion it was obliged to declare war on the Soviet Union and send a certain number of divisions to Polish territory to resist the Red Army. Clearly both states were more afraid of Soviet expansionism than of German expansionism.
If by "Germany and its allies" you mean Japan, then you forgot that on 15 September 1939 the USSR signed an unlimited ceasefire with Japan.
And in April of 1941 a non-agression pact was signed with Japan.
All attempts to create alliance with France, Britain and Poland failed, mostly because of Polish position not to sign any agreements with USSR.
I am not surprised that Poland rejected the Soviet "generous offer". The Soviet "generous proposition" was as follows:
The Soviet "offer of help" in case of Polish-German war included only demand ("proposition") of allowing the Red Army to "come in contact with the German army"
(this is accurate quotation) in Eastern Galicia, Suwalszczyzna (the region around the towns of Suwalki and Augustow) and Pomerania (= coastal areas of East Prussia). Not in other areas.
You do realize that allowing the Soviet "ally" to enter and march across Polish territory would mean de facto Soviet occupation. Was there any territory in the entire history of the Soviet Union, which - once entered - was ever peacefully abandoned by the Red Army? No. Was there any "ally" of the USSR which avoided "Sovietization" and becoming a puppet state of the "great mother Russia"? Probably no.
The fact is that entering into an alliance with the USSR (= accepting the Soviet demand of allowing the Red Army to march into Eastern Galicia, Suwalszczyzna and Pomerania) equals becoming puppet "vassal" of the USSR.
What REAL interest did Stalin have in proposing that "support" for Poland?
Was that proposition really about providing real support? Or just another political game of Stalin.
We know that Stalin's aim was to provoke an outbreak of war betwen Western Powers and the III Reich.
But how would supporting Poland help this to happen?
Moreover - Bolshevik crimes and their "reputation" from 1920 were still remembered in Poland.
Hitler, on the other hand, was - at that time - not yet renowned for his crimes.
If Poland accepted the Soviet "offer", annexation of Poland by the USSR would be very probable.
Of course from modern perspective Polish sufferings under the German occupation were not worth refusing the Soviet "generous offer". But as I already wrote - in 1939 it was the Soviet Union which had reputation of criminal state, not Nazi Germany. Nazi Germany gained reputation of criminal state only during the subsequent years.
In 1939 Poles did not consider Soviet leaders as ones who could be trusted. Rightly.
Domen said:
The Soviet "offer of help" in case of Polish-German war included only demand ("proposition") of allowing the Red Army to "come in contact with the German army" (this is accurate quotation) in Eastern Galicia, Suwalszczyzna (the region around the towns of Suwalki and Augustow) and Pomerania (= coastal areas of East Prussia). Not in other areas.
This means the Soviets were not interested in helping the Polish army to resist the German onslaught where it would be most urgent, most necessary (depending on strategic situation) - but only in some certain parts of Poland.
Strangely enough, these parts of Poland were the same as the Soviet Union annexed during further years:
Eastern Galicia and Suwalszczyzna were in the Soviet sphere of interests in the Ribbentrop-Molotov Pact.
Eastern Galicia = more or less the same territory as "Western Ukraine".
Eastern Galicia was annexed in September of 1939. Suwalszczyzna was not annexed only because on 28.09.1939 some changes were made in the initial plan of partitioning Europe from the Ribbentrop-Molotov Pact. According to the agreement of 28.09.1939 Lithuania was given to the USSR in exchange for Suwalszczyzna, while in the original Pact Lithuania was within the German sphere of interests and Suwalszczyzna within the Soviet.
Pomerania (meaning "coastal part of East Prussia" in this case) was annexed in 1945 (Kaliningrad Oblast).
This all proves, that Soviet alleged intentions of "helping Poland" were in fact dishonest.
All the Soviets wanted was probably to enter these territories, hold them until German army destroys Polish army elsewhere and then annex them, just like in real they annexed Eastern Poland and under the same pretext. After this happening, they would probably sign ceasefire with Germany or something (in case if they would ever declare war on Germany before - because "come in contact with the German army" may mean something else).
And the eventual "cooperation" of the Red Army with Polish Army - if any would even take place - would probably look just like the cooperation of Home Army (AK) with the Red Army against Germans looked like in 1944 / 1945. In case if you don't know, that cooperation was as follows: after joint Soviet-Polish combat against German forces and joint liberation of certain area from Nazi hands, Soviet forces were disarming and arresting Polish partisans...
Note also that "entering Suwalszczyzna" inevitably equals marching across entire "Western Belarus".
So if the Red Army was allowed to enter Suwalszczyzna, it would also enter Western Belarus for sure.
And - as we all know - "Western Belarus" was also annexed by the Soviet Union in 1939.
The state like Soviet Union simply was not and could not be Poland's trusted ally in 1939.
Even accepting Hitler's offer was more probable than accepting Stalin's offer. After all Poland had quite good international relations with Nazi Germany for a considerable period of time. Also most of Polish high-ranking officers considered the USSR a bigger threat for Poland's independence than Nazi Germany.
As for secret part of the agreement - yes, one of its purposes for USSR, was to return Western Belorussia and Ukraine territories
These territories were never possesed by the USSR before September of 1939.
which were annexed by Poland in 1920.
They were annexed by Poland in 1921 (Treaty of Riga), not in 1920.
Actually, it was not "one-handed" annexation by Poland. Also the USSR participated in dissolution of independent Ukrainian state.
Treaty of Riga was a
partition of Belarus and Ukraine (both eastern and western) between the USSR and Poland.
without agreement in June 1941 they would be much closer to Moscow.
They would be in a much more comfortable position. There was a powerful line of fortifications called Stalin Line across the old Soviet border. It was much more powerful than the newly-started in 1939 Molotov Line, construction of which was not yet finished before June 1941.
The "more terrain to retreat" advantage is not always an advantage - and in this case it clearly wasn't.
Especially considering that the entire retreat of the Red Army in the Summer / Autumn of 1941 ended in a terrible disaster - encirclement and subsequent destruction of tens of hundreds of thousands Soviet troops in a number of massive pockets.
In this situation, non-aggression agreement with Germany allowed to postpone German aggression by almost 2 years
The purpose of non-aggression pact for USSR was to postpone German possible attack for as long as possible.
Germany was not planning to invade the USSR in 1939. That's more than certain.
Hitler wanted to eliminate France and Great Britain first. He considered an alliance with Great Britain too.
The purpose of non-aggression pact (or rather its secret protocol in which the partition of Poland was agreed) for Germany was similar - to ensure that they would receive Soviet military help in their campaign against Poland and to ensure that Poland would not receive Soviet military help against them.
Without signing the Soviet-German pact before the invasion, Hitler would probably never risk to invade Poland.
and greatly increase military potential of Soviet army.
So I wonder why the full mobilization of the Red Army was halted after the Soviet-German pact was signed and after the subsequent defeat of Poland by German and Soviet forces?
Many Russian historians claim that the Red Army was not prepared at all for the German invasion in June of 1941 and that's why it collapses so rapidly during the first months of Barbarossa.
You claim the opposite - that the Red Army was prepared for the German invasion because it managed to "greatly increase its military potential", thanks to the pact with Germany in 1939.
So which is true? Was it prepared for the German attack in 1941 or not?