Molotov-Ribbentrop, The Effortpost
First we should acknowledge that this is often alluded to as, or even blatantly called, some kind of Soviet-German "alliance" which is incorrect. The short story is that the non-aggression pact was only signed after it was blatantly clear that the west would not act until Hitler invaded a large west friendly power and was otherwise quite willing to be friendly with fascists. The British/French hoped instead that the Fascists would focus on the USSR which they themselves had invaded and tried to destroy 20 years earlier -- the NAP was Russia's way of avoiding this since they clearly weren't ready to face the German/Italian and possibly Japanese army. An unhappy compromise, yes, but certainly not an alliance and certainly not their primary choice.
Now for a longer explanation I'll talk a bit about what led up to the NAP so you can have a better understanding.
The USSR's "Premature Anti-Fascism"
Whether or not we can criticize them in hindsight for thinking it could be done, it is a fact that the USSR spent the 30's trying to stop or delay the obviously upcoming war by attempting to get the western powers to ally with them against the Fascists.
In the early 30's they proposed a policy of complete disarmarment in hopes of de-escalating the situation but that and their draft for partial disarmament were voted down by the Western Powers.
The USSR also tried to push a policy of "collective security" and "indivisible peace" headed by ambassador Maxim Litvinov, who proposed an international antifascist alliance, basically trying to get a pact signed saying that the USSR and Western Powers would work together to stop Germany when they showed aggression. However, this was never agreed to by the west. It was becoming more and more apparent that the western Powers were not interested in an antifascist alliance. In fact I think you'll see that Britain and France (and somewhat the US) carried on a policy of appeasement and in some cases even outright collaboration with the Fascist powers as World War neared -- and that they hoped instead that Germany would go to war with the USSR and leave them out of it, killing two birds with one stone.
Western "Appeasement" and "Neutrality"
In 1936 the Fascist powers of Germany, Italy, and Japan had entered into an "Anti-Comintern Pact", basically an agreement to report to each other on the communists and to aid each other in fighting them if/when it came to it -- to the western powers this was a very good sign that they would attempt to attack the USSR rather than the west and its allies. At this same time the Spanish Civil War broke out -- the Western powers decided to remain "neutral" (the US still allowed companies to sell arms to the fascists and France/Britain actively had a blockade to prevent arms/soldiers from helping the republic). With France actively blocking them from sending supplies via land the USSR had to send aid to the Republic (weapons, planes, food etc) with ships attempting to run the blockades yet they still provided a sizable amount of aid as well as military advisers. The USSR also directed Communist parties around the world to organize and recruit the International Brigades and took in many refugees from Spain.
In 1937 Japan invaded China, an endeavor that would last until the end of WWII. Even though this threatened Imperialists interests to some degree and turned public opinion of their people against the Japanese the western powers (apart from the US providing China with some loan assistance) turned practically a blind eye to it for the first several years. You can get a glimpse of their attitude from the wikipedia page relating to this era
in July 1939, negotiations between Japanese Foreign Minister Arita Khatira and the British Ambassador in Tokyo, Robert Craigie, led to an agreement by which Great Britain recognized Japanese conquests in China. At the same time, the U.S. government extended a trade agreement with Japan for six months, then fully restored it. Under the agreement, Japan purchased trucks for the Kwantung Army, machine tools for aircraft factories, strategic materials (steel and scrap iron up to 16 October 1940, petrol and petroleum products up to 26 June 1941), and various other much-needed supplies.
In 1938 Germany carried out its "reunification" with Austria under the pretext of uniting all lands occupied by Germans. The British government under Chamberlain gave its approval and after the Germans marched in with no resistance the takeover was recognized as legitimate by Britain, France and the US.
The Munich Agreement (or Chamberlain-Hitler Pact)
Later the same year Germany turned its eyes to Czechoslovakia, demanding that the Sudetenland (a highly industrialized area with most of Czechoslovakia's defenses in it) be given to them. Britain and France at this point informed Czechoslovakia that they would not be giving them any military support (essentially breaking their previous pacts) and pressured them to give into the Nazis demands, the US supported this stance as well. At this point the USSR offered military aid (which was in fact going beyond what their pact required, which said that they only needed to help if France was aiding them as well)— they mobilized a large amount of troops tanks etc but the pseudo fascist government of Poland denied them passage and soon enough, in spite of the masses being against capitulation, the Czeckoslavakian government did not want to go to war without the aid of the west. Eventually Nazi Germany, Britain, France, and Italy signed the Munich Agreement (which the US played a part in organizing and approved of) which was basically them deciding for Czechoslovakia how the sudeten and other frontier territories were to be given to the Nazis as well as showing establishing a Western Non-intervention in certain actions Germany might take. Here you can see a map of how it was eventually divided up between Germany, Hungary, and Poland. In Czechoslovakia this treaty is called the "Munich Dictate"(since the country it was pertaining to wasn't invited) or the "Munich Betrayal". This was a pact of non-agression (and of dividing up a nation) between the Western powers and with the Nazis but you never see it called such (and you never will see it labelled as an alliance).
The Molotov-Ribbentrop Pact
In March 1939 the Nazis seized the rest of Czechoslovakia and seized land in Lithuania while Italy invaded Albania. This led to a slight change in policy of the west who at least opened up to discussion of a pact with the USSR. However, Britain/France's history of actions with the Nazis in relation to the USSR undoubtedly led the Soviets to be wary. This coupled with the fact that the western powers were unwilling to define exactly what mutual aid should be given, that they would indeed go so far to declare war if the USSR was attacked, or that they would help to defend the Baltic states led the USSR to suppose that they were insincere. The USSR had also received intelligence that Britain was still negotiating with the Nazis (they even continued this after the invasion of Poland leading to a "phony war" and still tried to sue for peace with the Nazis after the invasion of France up until at least mid-1940)— this would make it appear that the allies just were using the threat of an alliance with the USSR as a bargaining chip with the fascists. When, after long delays, the Soviets discovered that the British/French military missions sent didn't even have the authority to sign a military convention negotiations were broken off.
As we can see the USSR tried on numerous occasions to curtail the fascists and to ally with the western powers against them— conversely the western powers aided the Nazis in annexing territory and hoped to turn their hungry eyes toward the USSR, progressively worsening relations with the Soviets (who the west saw as the bigger threat until far too late) and helping to strengthen the growing Nazi empire.