I cannot provide information on all of the nations that would become the Allies, but I can talk about the abilities of the Red Army in relation to the Polish invasion.

The Soviets jointly invaded Poland with Nazi Germany in 1939 as part of a pre-organised plan between the two nations made at the same time of the Nazi-Soviet non-aggression pact of the same year, however this was kept secret until well after the end of the Second World War. The Soviet Red Army's performance during the invasion of Poland indicates that the USSR was by no means ready for organised warfare against Germany and the Wehrmacht.

One of the chief reasons for the Red Army's poor performance during the invasion of Poland was the poor state of its high command. The Great Terror of the latter stages of the 1930s (also known as Yezhovchina) decimated the Red Army's high command and its officer cadres. Of the 899 officers in the Soviet high command, 583 died.^1 During 1937 to 1938 Stalin’s purges of the military led to ‘the expulsion and execution of thousands of experienced officers’.^2 Indeed, around 40,000 military personnel were arrested during the Terror, ‘relatively few of whom survived the camps to be “rehabilitated” when war came.’^3 The inadequacies caused by the purge of the Soviet high command would not only affect the Red Army's performance in Poland, but also in the Soviet-Finnish War and Continuation War as well as during Operation Barbarossa, the German invasion of the Soviet Union in June 1941. In this way, though ostensibly successful, the Soviet invasion of Poland can be seen as a precursor in highlighting some of the causes of the Red Army's dramatic losses during the initial stages of the war with Germany later on. For the purposes of staying on track I will not go into the motivations behind Stalin's purges of the Red Army, but only the effects that they had in relation to your question.

The Terror inflicted a staggering amount of harm to the Soviet high command:

“3 out of 5 Marshals of the Soviet Union died, 3 out of 5 Army Commanders …., First Class, all 10 Second Class, 50 out of 57 Corps Commanders …., 154 out of 186 Divisional Commanders …, 401 out of 456 colonels, plus almost all corps and divisional commissars.”^4

It is not an overstatement to say that these purges crippled the Red Army not only physically but also strategically. One of the Marshals of the Soviet Union that was purged was Mikhail Tukhachevsky, perhaps one of the brightest minds in the Red Army. Tukhachevsky's ideas on the mechanisation of the Red Army (which Stalin and Voroshilov were deeply against) would post-humously prove to be vital as a basis for the tactics employed by the Red Army at Kursk in 1943. His execution was in hindsight a bitter blow to Soviet military strategy, as the loss of Tukhachevsky also meant the loss of ‘one of the Soviet Union's most innovative military theorists and strategists’; this dealt a severe blow to the Red Army, and the lack of implementation of Tukhachevsky’s ideas on military mechanisation can be seen to have had a disastrous effect on its performance in the imminent years following his death.^5 Indeed, only the year before his arrest, in January 1936, Tukhachevsky had made a speech to the Second Session of the Central Executive Committee of the USSR explicitly warning of the threat of Japanese and German rearmament, particularly their formation of tank and mechanised divisions as well as a powerful Luftwaffe.^6

Because of the executions of so many talented or experienced generals and officers, the organisation of the invasion of Poland was put in the charge of Kliment Voroshilov. Despite his ineptitude and military incompetence, Voroshilov came to command the Soviet military establishment under Stalin as Defence Commissar and a Marshal of the Soviet Union, helped largely by his affiliation with Stalin in the First Cavalry Army during the Russian Civil War. In fact, Voroshilov's military ineptitudes appear to be so large that the success of the Soviet invasion of Poland appears to be almost entirely due to factors outside his control. Indeed, 'Polish resistance was certainly far more limited than it might otherwise have been, given that the Wehrmacht had destroyed a significant proportion of the Polish armed forces before the Soviet invasion’ had even begun.^7 Indeed, the Red Army ‘encountered only isolated pockets of troops’ as ‘[t]he Poles were already broken by the German onslaught.’^8 Rather amusingly, Voroshilov’s invasion was also helped by the fact that Polish border guards in the East were ‘evidently confus[ed] on the ground over whether the Red Army was entering Poland as a conquering force or in fact in order to assist the Poles in the war against Germany.’^9

Thus, it is nigh-on completely implausible that the Soviets could have defeated Germany had they decided to go further than occupying east Poland in 1939, and it is therefore unlikely that a combined Allied effort against Germany would have been successful at that stage, particularly as Britain was also unprepared for a complete mobilisation against the Third Reich and the United States would equally unlikely to have been able to attack quickly.

Sources

1. Kochan, Lionel, and Keep, John, The Making of Modern Russia, 3rd ed., (London: Penguin, 1997), p. 394.

2. Whitewood, Peter, ‘Subversion in the Red Army and the Military Purge of 1937-1938’, in Europe-Asia Studies, vol. 67, no. 1, January 2015, pp. 102-122 (p. 102).

3. Kochan, and Keep, The Making of Modern Russia, p. 394.

4. Bellamy, Chris, Absolute War: Soviet Russia in the Second World War, (London: Macmillan, 2007), p. 46.

5. Budianksy, Stephen, ‘The Bolshevik who Believed in Tanks’, in World War II, vol. 26, no. 1, June 2011, p. 25.

6. ‘Speech by Comrade M. N. Tukhachevsky, Assistant People's Commissary for Defence, Marshal of the Soviet Union, made at the Second Session of the Central Executive Committee of the USSR on 15 January 1936.’, in The Slavonic and East European Review, vol. 14, no. 42, April 1936, pp. 694-701 (p. 695).

7. Hill, Alexander, ‘Voroshilov’s “Lightning” War – The Soviet Invasion of Poland, September 1939’, in Journal of Slavic Military Studies, vol. 27, no. 3, July 2014, pp. 404-419 (p. 406).

8. Bellamy, Absolute War, p. 59.

9. Hill, Alexander, ‘Voroshilov’s “Lightning” War’, p. 408.