I read in Gerhard Weinberg's book A World at Arms: A Global History of WW II that Molotov sent a note to the Germans in autumn 1940 as a feeler to gauge German interest in the Soviets joining the Tri-Partite Pact but never received a response as Hitler had already decided on the invasion. Is this true?
Is this true?
Yes and no. There definitly was an initiative from the german embassy in moskau to make the soviet union join the Tri-Partite Pact and making it into the Four-Partite Pact, if you want to say it like that.
It was meant as a secret addendum to the Molotov-Ribbentrop-Pact. When Molotov visited Berlin on the 12th of November 1940, Ribbentrop made the above meantioned proposal to Molotov who delivered it to Joseph Stalin.
Stalin however, who did not want the soviet union to end up in a war with britain responded with a note on the 25th of November 1940. The proposed conditions were unacceptable for Hitler, who didnt even bother to respond.
Now before you drive of into alternate history fantasies, the actual impact even if the soviet union had joined the Tri-Partie Pact is highly debated. What is not debated however, is that the planing for operation Barbarossa was not affected in any way by these "negotiations".
Sources:
Ingeborg Fleischhauer, Diplomatischer Widerstand gegen „Unternehmen Barbarossa“: Die Friedensbemühungen der Deutschen Botschaft Moskau 1939–1941.
Wolfgang Michalka, Ribbentrop und die deutsche Weltpolitik. Außenpolitische Konzeptionen und Entscheidungsprozesse im Dritten Reich.
So Sven Allard, Stalin und Hitler. Die sowjetische Außenpolitik 1930–1941
Please excuse that my sources are all german. You should be able to find alot of sources in your native language.