Nothing is more frustrating than reading history and having well-paid historians repeat the same untruths over and over. One of the most common is that the Nazi-Soviet pact was Stalin's biggest mistake and that Stalin "trusted" Hitler. Its written as if it was accepted truth instead of a myth.
1. Anyone who wants the background on this subject should read:
a) Read and Fischer - The Deadly Embrace
b) Molotov Remembers
c) Khrushchev's Memoirs
2. The situation in September 1939 was this. The USSR was in no position to go to war.
First, not only was the Red Army Officer Corps recovering the Stalin's purges, the Red Army and Air Force had only obsolete tanks and aircraft. Tanks capable ( t-34 and KV-1) of fighting the German PZ III and IV were not available in quantity until June 1941. Russian Single engine fighters equal to the ME-109 were not available until April 1941 (Yak-1 and LAAG-3) .
Secondly, the 2 years breathing space gave the USSR a chance to call up millions of men and further mechanize and increase its tank strength. The Red Army increased from Sept 1939 to June 22, 141 as follows:
Sept 1939 - 96
June 1941 -198
Sept 1939 - 1.5 million
June 1941 - 5.4 million
Sept 1939 - 4
June 1941 - 29
Sept 1939 - 1
June 1941 - 31
Sept 1939 - 0
June 1941 - 61
The degree of mechanization from Sept 1939 to June 1941 is shown not only by the above figures but by the fact that the USSR produced 250,000 trucks from Sept 1939 to June 1941 and the Red Army had 300,000 trucks at the start of the war.
Thirdly, the delay allowed the Soviets to build and move important war plants in the Urals and 500 and 600 miles away from the Border. This along with the Evacuation plan put into effect in June-Dec 1941 allowed the Soviets to survive the initial German assault and continue war production. Further, this evacuation was only effective because the soviets built almost 2, 000 locomotives from 1949-1941 increasing their number from 16, 000 to 18,000.
Fourth, I haven't even covered the massive increase in explosive, ammunition, anti-aircraft, and anti-tank production that occurred during this period.