He did, the German-Soviet pact was done not as much as an alliance but more so as both Hitler and Stalin knew they needed to stall in order to prepare for war. Stalin began the purges and transformed the army into loyal soldiers while Hitler gathered resources and set up a front and offensive line before the attack, as it turns out, moving troops and resources takes a lot less time than purging an army and starting back from the top, so Hitler was both ready and able to fight first. Once Stalin was prepared you see how the war changed. Neither Hitler nor Stalin knew or wanted the pact to be a permanent truce and they both knew the other felt the same. However they both also gained in multiple ways from it. Splitting Poland in two and Stalin getting (most) of the 3 Baltic sisters. (Memel went to Germany). Germany also told the Soviets they could have free reign over Finland. But that didn’t work out for them.
Here's a post of mine from another thread.
As others have said, Stalin knew that war was going to happen eventually. What he assumed, though, was that Hitler wasn't crazy enough to open a second front against the Soviets (at least not at the outset). He thought that the Germans would drive westward and only turn eastward later, to avoid a two-front war. In fact, at the outset of the Molotov-Ribbentrop pact, he was probably very confident in the Soviet position - he had averted war for a time with the Nazi's, who he predicted would drive westward and weaken capitalist states, while at the same time taking back territory in eastern Europe lost over the past couple decades during war and revolution.
He also didn't expect the Germans to advance so quickly. The swift surrender of France was alarming to Stalin and threw into question his entire strategy towards the Germans; if the west is conquered too quickly, then the Nazi's would turn eastward much sooner. So even before the front was opened he was already worrying about impending war. The Soviets accelerated their plans for "Sovietization" of the Baltic states. Further, the disaster of the Winter War in Finland not only showed the poor state of the Soviet military, but also was coming close to a German conflict, as the Nazi's drove eastward through Norway, something he obviously wanted to avoid.
In November 1940, in a meeting with Molotov, Hitler invited the USSR to become a partner in the Tripartite Pact, effectively offering them to join the axis. This of course wasn't a reality, given that both sides knew war between them was coming, but both wanted to buy time. Both sides negotiated on how to come to an agreement, but ultimately Hitler approved the invasion of the USSR in December. During this entire period the Soviet leadership was working to get the military in a war-ready state.
The Germans attacked in June 1941. The night before, a German sergeant defected and warned the Soviets, who weren't sure if he was telling the truth or a provocateur. Despite the request of his generals to make the troops combat ready, Stalin denied that an invasion was imminent and asserted that it was possibly the latter.
Stalin was informed shortly after 3am that the Germans had invaded. Military leadership arrived to meet with him shortly thereafter, at which he continued to express doubt about the invasion, positing that it could merely be a provocation by German generals and that "Hitler surely doesn't know about this." He sent Molotov to meet with the German ambassador and refused to order a counterstrike until he returned.
Molotov returned. His meeting with the Ambassador did not go well (obviously). The German claim was that they had to take countermeasures in response to the aggressive military build-up on the Soviet's side. They had clearly not needed much of an excuse to invade. In response to Molotov's account of the meeting, Zhukov claims Stalin "silently dropped into his chair and became immersed in thought. A long and painful pause ensued." He concluded that a counterattack was acceptable as long as the troops didn't violate German borders. This was four hours after the invasion commenced. Zhukov continues:
During the first day he was not able to really take himself in hand and get a firm grip on events. The shock to I.V. Stalin caused by the enemy invasion was so strong that his voice even became softer and his instructions on organizing the military effort were not always appropriate to the situation."
Chadaev later recalled:
Early on the morning of 22 June I caught sight of Stalin in the corridor. He had arrived at work after a brief sleep. He looked tired, worn out, and sad. His pockmarked face was sunken. You could see he was depressed.
In short, the reason that Stalin was so surprised was because he was so committed to both a strategy and an idea of Hitler that were both wrong. His strategy for dealing with the Germans was to enter into an agreement with them, to ensure that they didn't drive eastward. This was precluded by the Nazi's swift westward advance through Europe and Hitler's foolish idea to open a second eastern front (something Stalin didn't think him stupid enough to do). His assessment of Hitler was that of a relatively reasonable man, who he didn't think would subvert the agreements made between the two countries as easily as he did. Remember that even in November and December as Hitler was preparing to formally approve the invasion of the USSR, Stalin and Molotov were still attempting to negotiate with him on the Tripartite Pact, Finnish occupation and Eastern European issues (Romanian oil fields, for example). So when the invasion commenced, even though he had a pretty good idea of it happening at some point in the future, he didn't think it would come either as soon as it did, or in the manner it did. His denial of the invasion, claiming the possibility of a provocation by German generals and not Hitler's order, is evidence of his commitment to this idea and his disbelief at the notion that he was wrong.
Remember also that Stalin was just never wrong. He never admitted wrongdoing and hardly changed his mind on anything. The only times he ever changed his mind on issues were when he was faced with a physical impossibility, and in every instance of this he did so very slowly and grudgingly (and of course never admitting so). This is a great example of that.
Source: Stalin, New Biography of A Dictator. Khlevniuk, Oleg V.
Mentioning previous answers is intended as a starting point for discussion, not to discourage any further discussion. Follow-on questions, data, and disagreements are welcome.
I am astonished that I can't find anything in the FAQ for this!
I have found previous discussions here, but they tend to repeat the same conventional wisdom, without citing sources, going into detail, debunking anything, ... That could be a result of unquestioned truth -- "what was the date of D-Day in Normandy?" "6 June 1944" doesn't require discussion. Or it could be myth, like "Stalin locked himself in a room for [time period]".
The onez that seems most interesting are
"Was Russia/Stalin truly hoping to share the Europe with Germany/Hitler or was Stalin playing a waiting game?" at https://www.reddit.com/r/AskHistorians/comments/34gqic/was_russiastalin_truly_hoping_to_share_the_europe/ , with interesting notes by /u/MrMarbles2000 , /u/blackbird17k , and Naugrith quoting their answer from the next discussion, to wit:
"Without hindsight was Stalin's reasoning for believing Hitler wouldn't invade reasonable?" at https://www.reddit.com/r/AskHistorians/comments/2xo4zx/without_hindsight_was_stalins_reasoning_for/ , with replies by /u/Naugrith , MrMarbles2000, and NoAstronomer.
The conventional reply is that Stalin didn't trust Hitler at all, but hadn't seen any good way to oppose him earlier, hadn't trusted Britain and France, hoped that Germany and Britain would wear each other down more, wanted more time to build up arms, thought that the last-minute buildup was just trying to put pressure on him, and didn't think that Hitler would be so stupid as to open a second front. Mentioned in
"Logistically, why was Stalin surprised by Operation Barbarossa? Did his spies not notice hundreds and thousands of German troops massing at the border?" at https://www.reddit.com/r/AskHistorians/comments/2ygfga/logistically_why_was_stalin_surprised_by/
"Why did Stalin not believe Hitler would betray him when he was so paranoid about almost everything else?" at https://www.reddit.com/r/AskHistorians/comments/4oq3fz/why_did_stalin_not_believe_hitler_would_betray/
"Why did Stalin choose to trust Hitler before the Second World War?" at https://www.reddit.com/r/AskHistorians/comments/3px35z/why_did_stalin_choose_to_trust_hitler_before_the/
"What was Stalin's reaction when barbarossa started?" at https://www.reddit.com/r/AskHistorians/comments/6gmss9/what_was_stalins_reaction_when_barbarossa_started/