Russia/USSR had three scores to settle in WWII, and they succeeded in settling two of them. The three scores were, in chronological order, the Japanese beating the Czarist Navy in 1905, the British getting control of the middle east oil fields in WWI, and the Nazis reneging on their neutrality pact.
My guess, and it's only a guess, is that Stalin thought that after he signed the neutrality pact, Hitler would concentrate on the west until Britain was brought under Nazi hegemony, perhaps completely conquered. In the meantime, Stalin could take the oil fields for himself, and then catch the Japanese flat-footed in the Kuriles in 1941, perhaps even taking Hokkaido. He could then decide how to handle Hitler, whether to take him for an ally against the United States with Stalin taking Alaska while Hitler took over the Atlantic and gained allies in Latin America, or to fight Hitler head-on in Poland while convincing Japan to lay off Hawaii, and getting America into the European war.
All that went out the window when Britain didn't buckle under the Nazi attacks, Russia didn't get the oil fields, and then Hitler reneged and invaded in June 1941: that put avenging against Japan on the back burner, and so Stalin claimed neutrality with Japan during 99% of the war. This was because Stalin had bigger fish to fry: he had to do what Alexander I had done to Napoleon 130 years earlier, which was to make Hitler fight for every square inch of ground, stretch out the enemy supply lines to the breaking point, then let the winter conquer the enemy. Just as Alexander and Napoleon went from allies to moral enemies, so did Stalin and Hitler.
Stalin's mistake with Japan was that he actually kept his word given at Yalta, and waited three months from the end of the European war to invade. He could just as easily have broken his word--he was a Communist after all, integrity is not their strong suit--and invaded the Kuriles and Hokkaido in July, presenting Truman and Churchill a fait accompli at Potsdam. Why he didn't is one of those questions for which there will never be a good answer.
I have a slightly different take on Stalin’s motives and actions. While I don’t disagree that Stalin had those three goals in mind, I want to add a fourth one, which is the Sovietization of Europe. Not so much to “spread the revolution” but as an expansion of Soviet power. And to some extent, for security too. The rise of Hitler and the Non-Aggression Pact were keys to his pre-June 1941 policies. Stalin had some opportunities to pursue the three goals you mentioned, but he declined because at the time the opportunities arose, the downside was too great. The situation of 1939-1941 presented more dangers to the USSR than opportunities, and Stalin played a very cautious game at this time. He had plans to turn it to his advantage later, if only he had the time to get there without a major war before he was ready for it. And it all revolved around his uneasy relationship with Adolph Hitler, a man he never met.
By the late 1930s, it was becoming apparent that Nazi Germany under Adolph Hitler was going to foment another major war. Stalin, who had been quiescent in international diplomacy, decided it was in the security interests of the USSR to become more of a player. To be a player with credibility and for security he needed to overhaul and expand the Red Army. The expansion part was done by rapidly mobilizing and conscripting much of the eligible male population of the USSR. The Red Army had maintained a stable force level of about 1.5 million men for most of the late 1920s and 1930s. Starting in 1937 or 1938, it began a rapid expansion, reaching a total of about 5 million in June 1941. Of course, such an expansion had its problems. In order to be a proper army, those men must be trained, equipped, supplied, and led. (While the rapid mobilization bloated the Red Army, the mobilization administrative machinery created by Marshal Boris Shaposhnikov remained in place in 1941. It allowed the Red Army to create new armies faster than the Germans could destroy them, and saved the USSR in the fall of 1941.)
In the equipment area, Soviet industry was just beginning to turn out some fine new weapons systems to equip the army, such as the T-34 tank, but in 1939, those systems were still in the development stage. Even by 1941 the Red Army was mainly equipped with obsolescent and poorly maintained weapons. Also, the level of logistic support services were not in place to support the bloated force structure.
Which gets us to the problem of leadership. By expanding the numbers of men, officers had to be rapidly promoted one or two grades beyond their experience to fill the officer billets created in the new formations. Stalin added another problem on his own. To Stalin, “overhauling” the Red Army, meant purging its leadership of anyone who might be a threat to Stalin’s rule. So he purged the officer corps with extreme prejudice. While it is true that he cleared out a lot of “dead wood” in older officers who would not have fared well in the fast-paced combat of World War 2, he also cleared out a lot of competent officers who could have learned their new responsibilities. So now you have a lot more billets to fill with far fewer officers. And those officers who survived tend not to be the Gheorgy Zhukovs; they are Dmtri Pavlov, the indecisive and unlucky commander of Western Front, summarily executed in July 1941 for not resisting the fascist invader with sufficient vigor. The Red Army officers are afraid of taking any sort of initiative, as the wrong initiative leads to the firing squad. So they will only follow written orders to the letter. That’s no way to lead an army against German Blitzkrieg. The general shortage of competent officers also led to another problem. Since the command billets had to be filled, the staff positions suffered. Many staff positions went unfilled, or were filled by men who had little idea of what staff work was. The deficit in basic operational ability was marked when the supreme test came.
The upshot is that in 1939, with war looming, Stalin knows his army isn’t ready to fight Hitler. But he was willing to consider opposing Hitler in an alliance with Britain and France, and engaged in some negotiations with an Anglo-French military mission. The problem was that the British and French were not serious about cutting a deal with the USSR, nor were they serious about fighting Hitler. It became quite apparent to Stalin that the Anglo-French strategy was to defeat Hitler by sitting out the war behind the Maginot Line and let the USSR defeat Germany at the cost of copious amounts of Russian blood. Stalin had no interest in this.
So Stalin cut his deal with Hitler. Stalin figured Germany, France and Britain would exhaust themselves in another rerun of World War 1. In the meantime, he would expand some buffer zones in Central Europe (the Secret Protocol to the Non-Aggression Pact), and would buy time to get his army in shape. Then he would clean up in Europe when both sides were ready for Red Revolution as they had been in 1918. Of course, he miscalculated. France caved in six weeks, the British were isolated...and so was the USSR, left with no allies and facing the largest, best trained best led and most experienced army in Europe.
Stalin’s new strategy after the Fall of France was then to adhere strictly to the Non-Aggression Pact in hopes he could squeeze enough time out of it. Stalin knew his army was not ready to face the Germans in 1941. In addition to all of the problems listed above, it was also occupying forward positions in newly occupied hostile territory without any adequate fortifications or supporting installations. Stalin also realizes he cannot make any new enemies at this point. So when Molotov visits Berlin in November, 1940, Hitler dangled the idea of the USSR moving southward against British Asian interests, which included India and the Perisan Gulf. Molotov demurred. Maybe Stalin was resentful of the British taking those oil fields after World War 1, but given the opportunity to take them he passed.
One reason Stalin passed on the opportunity is that it now suited him to keep Britain in the war against Germany. So long as the Royal Navy maintained a blockade of the Continent, the only way Germany could obtain raw materials was from and through the USSR. In addition to the near term belief that such reliance would keep Hitler from attacking him, Stalin had a longer term plan in mind. By 1942, or certainly by 1943, Stalin believed the balance of power would shift away from Germany and toward the USSR. The Red Army expansion and modernization would be complete, it would be a large, well-equipped, well trained and competently led force and would no longer be vulnerable to Germany. At that point Stalin would be able to engage in political blackmail of Nazi Germany. He could start demanding “concessions” in exchange for continued access to raw materials. In the end, Hitler would become his vassal. If Britain drops out of the war, that strategy goes out the window. With maritime access to raw materials, Hitler has no need of the Non-Aggression Pact and is free to turn on the USSR. Stalin also believed Hitler would not engage in a two-front war, as that strategic situation doomed Germany in World War 1. As long as Britain remains a belligerent, Stalin believed Hitler would not attack.
In any event, Stalin was wrong, his strategy failed, and Hitler turned on him in June 1941 The titanic struggle of the Russo-German War was on. I believe that Hitler sensed what Stalin’s end game was, and knew he had to strike first. The situation would never be more favorable to do so than it was in 1941. Germany may or may not have been doomed to lose from the outset, but it was a gamble they had to take. The irony here is that by being bogged down in their prolonged bitter struggle, Stalin wound up doing what the British wanted him to do and he had sought to avoid. Britain sat secure behind the English Channel, and watch the USSR defeat Nazi Germany at the cost of copious amounts of Russian blood.
Having won that struggle, Stalin is in position to accomplish most of his goals. Or can see the strategy playing out whereby he can do so. The Sovietization of Europe from Stettin to Trieste is an inevitability, as it was inevitable no later than June 22, 1941. The Americans have begged him into entering the war against Japan; Stalin is counting on Japan lasting long enough to allow him to replace the Japanese in Manchuria just as he had replaced the Germans in Eastern Europe. The British Empire is clearly in decline, and the United States through Truman’s speech today makes it apparent the United States will not take over British interests. It also appears the United States military occupation and presence in Europe will only be a token one. Thus, the USSR will be free to fill the power vacuum in the Persian Gulf.
Over all of this lies what Stalin believes is the decisive chip in the game of global power politics: The Red Army. It has become the force he wanted it to be, and he believes it’s might allows him to dictate events in Europe, Asia and even Africa. The Soviet people believe this, too. They have suffered greatly, but built a dominant war machine. The world will now have to listen to and bow to the interests of the mighty Soviet Union.
That world view will only last for another sixteen days.