Before the Nazi invasion of Poland there was in Moscow a negotiation for an Anglo-French-Russian alliance against the Nazis. What do we know about it? Why did it fail? Why did the Russians invite Ribbentrop to Moscow while they were still negotiating with the British and the French?

Because the British and the French wasn't that serious about it even if Stalin was. Stalin's sincere attempts to establish a revived Triple Entente was met by what he perceived as (probably correctly) rebuff from the British and French.

For example, in June 1939 when Molotov asked for concessions from the British on the Baltic states which he viewed as possible launching grounds for attacks on the USSR, the British refused him. Stalin was stunned by this, it was incomprehensible why a country whose empire covers a quarter of the globe seem to be unwilling to concede on three small countries bordering the USSR and hundreds of miles from the UK which he viewed as necessary for Soviet security. When domestic pressure compelled Chamberlain the same month to sent someone to Moscow to accelerate the negotiations, Chamberlain opted not to send foreign secretary Halifax nor ex-foreign secretary Anthony Eden but William Strang who was but a foreign office functionary. In August when the British did send a mission to negotiate with the Soviets, the Admiral in charge of the mission did not even have written authority stating that he had permission negotiate on behalf of the British government.

At the same time, the excellent Soviet spy network (including the infamous Guy Burgess of the Cambridge five) in UK were reporting that the British armed forces chief of staff did not believe that a Soviet alliance was necessary, and that Chamberlain’s advisors were also openly saying that a Russian alliance is unnecessary. They were also reporting to Stalin that the British were meeting with the Germans for negotiations, and that Chamberlain was offering Hitler concessions. Stalin had became convinced that the British were playing a double game, and that they were really interested in signing on to some kind of an alliance or non-aggression agreement with Hitler in order for Germany to turn on the USSR.

Stalin did not much differentiate between the democracies and the Fascists states: viewing both as simply "capitalist powers" and was nervous that a "pan-capitalist alliance" would be formed to destroy the Soviet Union. The response for that and to advance the cause of Socialism, Stalin (a true believer) wanted to pit the capitalist powers against each other. Because not only only does this enhance security for the Soviet state, but just as the "imperialist" WWI give rise to the Bolshevik revolution he believed that another long, drawn out war might lead to the rise of Communist revolutions in the combatant's countries.

It was really parallel to the game he believed that the British were playing against him, just as he knew that the British and Germans were negotiating with each other, he too will negotiate with Germany while negotiating with the Anglo-French. For that matter, it was similar to to the game the Germans were playing, one of the reasons why Nazi Germany was willing to negotiate with the Soviets is because -they- were convinced that Stalin might soon reach an agreement with the British. Thus the offer to Stalin was a way to preempt a possible Soviet-British pact.

At the same time, Ribbentrop and Nazi Germany proved to be better negotiating partners than the British, with Ribbentrop willing to concede to large number of Soviet demands over spheres of influence: over most of the Baltic states, over Romanian Bessarabia, over eastern Poland and over Finland, and mutually beneficial economic exchanges. The Nazis and the Soviets also got along personally, with one of Ribbentropp's entourage recalling later that it "felt as though as he was with old (Nazi) party comrades".

The Nazis in other words, were both serious about negotiating and were willing to give Stalin what he wanted. The terms were so favorable to the Soviets there was real discussions that Stalin might have formed a “pact of steel” with Hitler after the fall of France had Hitler given a few more concessions. Thus there was both realpolitik and ideological rationality for the Molotov-Ribbentrop pact as oppose to an anti-Fascism pact.

Source: Stalin Volume II, Waiting for Hitler by Stephen Kotkin