The Soviet Union was never allied with Hitler. The Molotov-Ribbentrop Pact was a treaty, not an alliance. The United Kingdom and France both signed similar non-aggression pacts with Germany in 1938, Denmark signed one in May of 1939, and Estonia and Latvia in June of 1939. The German-Soviet pact wasn’t signed until August of that year, making the Soviet treaty the last made with Germany, while everyone else was falling over themselves to make deals with the Nazis. And Belgium adopted a policy of neutrality towards Germany in 1936, and refused to militarily coordinate with Britain and France.
I’ll also say that, far from the now widely accepted historical fiction of appeasement, the British and French were actively supporting and encouraging Nazi expansionism and aggression. They essentially browbeat the Czechs at Munich into accepting the German annexation of their industrial regions, and the Poles and Hungarians also took part in carving off pieces of the country for themselves. Hitler was viewed as a useful tool, an asset that could be worked with. He was allowed to blatantly violate the Versailles Treaty, allowed to retake the Rhineland, allowed to annex the industrial areas of Czechoslovakia. Not as appeasement in the hope that he would be satisfied, but because it was believed a militarily and industrially strong Germany would be a useful bulwark against the dread threat of Communism. After the technical start of hostilities after the German invasion of Poland there followed a full eight months of ‘Phoney War’ in which neither Britain nor France took any significant action, instead waiting to see whether Hitler would continue east. They allowed him to invade Denmark, and only took action when he started on Norway, aid that they abandoned once he invaded France. It was only after the invasion of France that Chamberlain was finally forced to resign.
Chamberlains failure wasn’t one of appeasement, it was one of control and believing Hitler could be directed eastward. Chamberlain was forced out when it became clear he had lost control of his pet dictator, at which point the British upper-classes, who had largely supported this policy, panicked and put one of the few Conservatives who had consistently warned of the threat Hitler represented, Churchill, in charge. Appeasement is a post facto fiction created by Churchill and others to distract from the reality that they were trying to control Hitler and Germany by scapegoating Chamberlain for pursuing a failed policy that in fact never existed in the first place. Communism was the ‘great enemy’. Before WW2 it was all ‘communism, communism, communism’, and not long after the fighting ended the Cold War was begun. Hitler wasn’t a great evil (though he was that) that united the world because he simply couldn’t be tolerated, he was a dog who turned on his masters and that they were forced to crush, in the process allying with the ‘great enemy’ they had hoped to direct their pet against in the first place.