Greetings! Welcome to AskHistorians, and I do believe that you have asked a proper question as per the guidelines of this sub (those can be reviewed here, so you are up-to-speed on them). This is a most interesting question and it does speak volumes about why the Anglo-French-Soviet negotiations do not appear to be in mainstream, public, historical narratives of 1939. But in truth, in historical academia the failure of this proposed pact has been written about and analysed many times over, as it is definitely important to the overall narrative of the Second World War to understand why the Molotov-Ribbentrop Pact was successful and this Anglo-French-Soviet pact was not. As a side-note, you might find this thread I weighed in on about why the Molotov-Ribbentrop Pact worked useful as another source for this question. Let's begin.

Setting the Scene

In August 1939, the Moscow Mission by Britain and France failed spectacularly; with negotiations on a proposed Anglo-Franco-Soviet Alliance getting nowhere. The problems were apparent from the outset. Here's historian Michael Jabara Carley on the matter:

"The talks seemed of so little import that Halifax [British Secretary of State for Foreign Affairs] had 'scarcely perused' British instructions. Concerns about Soviet impatience if the British dragged out the talks were shrugged off. And the British delegation was instructed to avoid discussion of Soviet aid to Poland and Romania; the Soviet Union would have to negotiate directly with the Polish and Romanian governments".

The key problem was that France and Britain were both unwilling to accept Moscow's demands for an "ironclad military alliance". Stalin wanted a conditional collective security agreement, in which the Anglo-French government could support the Soviet Union in the eventuality of German invasion (a most astute prediction). After the failures of these talks with the Western Allies, it appeared as though Stalin's best option (ironically) lay in making a deal with Adolf Hitler and Nazi Germany. In addition to the greater likelihood that Hitler's diplomats would actually take negotiations seriously, the Soviet leader believed that neutralising the German threat directly would not only secure his country's precarious military isolation, but also give the Red Army precious time to recover after the disastrous "Great Purges" which had decimated its senior command. The rest as you likely know, is history.

That at least, is the basic overview of events and motives surrounding the "Western Betrayal" of the Soviets in 1939 (if we can even term it that).

Dissecting the Question

So, on to the deeper question at hand: why had Chamberlain (Prime Minister and architect of appeasement) and Churchill (who was by 1939 First Lord of the Admiralty) failed to press onwards with this Anglo-French-Soviet Pact? The answer is far more complex than a single response could ever hope to flesh out, but I will attempt to give an overview of the main points here.

Firstly, there was the ideological fear. The British government, ever since the Bolshevik Revolution of 1917, had feared the Communist giant which had overthrown their Tsarist ally in the First World War. To many, Churchill and Chamberlain among them, the Bolsheviks were the equivalent of the devil, and Hitler's regime was no worse (and even at times perceived as better) than Stalin's one. Even Chamberlain's ministers and advisers were hostile towards the Soviets, despite the interesting fact that much of their citizens actually preferred allying with the Soviets against Germany than attempting to appease them once more (though this opinion may have been more to do with pre-empting a disastrous war than actual support for the USSR). Jabara Carley on this conundrum:

"Halifax, a young anti-bolshevik 'die-hard' in the early 1920s, was said to regard the USSR as 'the anti-Christ'. Strang, head of the Central department, thought that Chamberlain opposed a Soviet alliance because it would signal the end of appeasement; he said 'all at No. 10 are anti-Soviet'. Halifax explained to the French that 'half the British population' held the Soviet Union responsible as much as the Nazis for 'all the troubles of the last 10 years'

The French were also weary of the Communists in Moscow, though they had by 1939 grown accustomed to dealing with left-wing parties in their own assemblies (and Prime Minister Edourad Daladier had silenced that internal threat by August 1939). In fact, the French seemed to be more concerned with ensuring that if an alliance could not be agreed upon, then it was the Soviets who should receive the blame for the failure, rather than the Western democracies. However, the French also (and perhaps mistakenly) decided to "go with Britain" on many of the points when negotiating the Soviets. They entrusted the Foreign Office to do much of the correspondence between the Allies and Moscow, whilst giving their support if certain conditions which the French believed ideal (such as not having to commit to a military alliance just yet), and generally waiting for the British to act on their behalf.

This stagnation and delaying was most upsetting to Stalin and Molotov, who were also trying to see whether the Germans or the Anglo-French were the better party to be doing business with.

Secondly, there was the condition of military intervention. The British and French were reluctant to commit their militaries in support of the Soviets, as that would clearly upset the Anti-Bolsheviks which occupied both governments, not too mention drag down public support for what would inevitably be a coming war. This point is actually interesting in itself, as Chamberlain also remarked that if it came to war with Germany, it would be the Soviets who could do the brunt of the fighting, as the British army consisted of only four divisions which could be spared for a European Expeditionary Force (he famously described this as "two now then two later".) The French also understood this benefit of an alliance, but their country had just suffered (and was still recovering) from the all-too traumatic and evident memory of the War to End All Wars. In order to address this problem, Chamberlain and his cabinet decided that Anglo-French military would only come if:

• "Indirect aggression" threatened either of the three powers, and the definition of that term was so ambiguous that the Foreign Office told Reginald Drax (the head of the Moscow Mission in August 1939) that negotiations should deliberately be slowed down to ensure Soviet acceptance of the British definition.
• The League of Nations had failed in intervening in German aggression (an eventuality which was practically guaranteed, since Germany had already left the League and therefore was not entitled at all to abide by its word).

These conditions would take more time than was ideal to negotiate and agree upon, which to Chamberlain and his cabinet was perfectly fine, as they (wrongly) believed that the longer the Soviets were kept busy negotiating with them, they would not dare begin negotiations with Germany, and Hitler would not begin his expansion drive again for fear of adding more weight to an Anglo-French-Soviet alliance.

Thirdly, we have the question of Romania and Poland. The Soviets wanted the British and French to negotiate with the Polish and Romanian governments as well, on the condition that Red Army troops be allowed to pass through their territory in order to combat the German threat. Neither the Romanians nor the Poles wanted this (after all, that same Red Army had invaded Poland in 1920, and the Romanians were also not particularly fond of their Communist neighbours). There was also the question of the Baltic nations, whose compliance Stalin and Molotov also wanted to secure, lest they become allies (willing or otherwise) of the Germans instead (and frankly, Poland and Romania were already difficult enough for the Anglo-French diplomats to collaborate with, the Baltic nations were a nightmare). As Jabara notes (perhaps somewhat logically though with a touch of exaggeration): "They preferred would a year of Nazi occupation to a day of Soviet - which was what worried the Soviet government."