Its not easy to speculate about what a historical figure may have thought, only to study what they did and said.
The war to defeat Hitler's Germany raged for six years, and it took the allied strengths of the so-called 'Big Three' of the USSR, America and Britain, to finally defeat Germany. At the end of the war, Germany, the aggressor, remained occupied by the three powers (and a resurgent France), its cities in ruins, and its economy in tatters. Britain, the old imperial power found itself nearly bankrupt after six years of war. America, economically the strongest of the big three, found itself lending supplies to both of the allies, and with the end of the war, loaning money to its allies as well as the liberated countries of Europe. The Soviets, unlike their allies wer alone in suffering the rigours of a horrific invasion, its economy mauled by German aggression and an unheard of proportion of its people slaughtered, an invasion which only by gargantuan effort had it successfully repelled.
Throughout the conflict, the allies, despite their ideological differences, optimistically maintained their alliance, hoping for its continuation after the peace. During the war, the allies were careful to maintain amicable relations: Churchill, staunch opponent of Communism commented at a dinner during the Tehran conference, in 1943 that the 'complexion of the world was changing and that a common meeting ground might be found for the different colors [sic].' Stalin is reported to reply by saying that even Churchill's complexion may be getting 'a little pinker'.In 1943 the alliance between the USSR, Britain and America was vital to victory, Churchill's comment shows the Anglo-American desire to maintain military cooperation with the Soviet Union despite any political differences. The 1943 conferences' main purpose was not to address conflicting political interests in a defeated Germany, but instead to concentrate on the coordination of allied operations within Europe, at Potsdam, in 1945 it was assumed that the allies could continue their collaboration into peacetime.
The fate of a defeated Germany was barely considered by the allies at this point, post war plans had to wait until victory became a firm reality, and prior to the second Quebec conference in 1944 the US president reportedly had 'no real plan for the management of occupied Germany.' Stalin, forever paranoid was primarily interested in 'arrangements that would give the Soviet Union secure borders.' His desire for secure borders however, was not coupled with a comprehensive plan for German division, and he was reluctant to propose any particular policy instead endeavouring to discover what his allies had in mind, but he did favour partition. British plans were mostly concerned with ensuring that Germany would not rise to begin again another war, and the maintenance of the alliance was considered important for the maintenance of the balance of power in Europe.Henry J Morgenthau, United States Secretary of the Treasury, proposed a partition of Germany which would be combined with forced de-industrialisation to prevent any resurgence of German aggression, the plan was criticised by the committee of Post-War programs, in a memorandum to Roosevelt, as an impractical solution, as the division and de-industrialisation would have to be maintained by military force, and ultimately, would lead to a new nationalistic desire to reunite.
Like military policy, potential occupied policy was strongly influenced by the interest of maintaining the alliance, Eden, and some of the British foreign office felt that a 'soft peace' towards Germany would only arouse Russian enmity. General Eisenhower felt that the demands for a 'soft peace' were influenced by those who 'wanted to make Germany a bulwark against Russia' and instead believed that Russia 'had all she could digest' and advocated letting the 'Germans stew in their own juice.' It was also at the second Quebec conference that allied occupation policy was agreed, specifically the zonal division of Germany, zones which would form the eventual borders of a divided Germany.
The problem then began with the cessation of the conflict. The Allies had never come to a conclusion of how to occupy Germany, they had agreed upon zones of occupation, but they had not agreed upon how they would administer those zones, and no coherent policy had emerged on how to deal with reconstruction and reparations. As it transpired, the Americans did not adopt the Morgenthau plan, whilst the British administration had generally been in favour of allowing limited reconstruction. American and British pressure also convinced the French to abandon their more stringent reparations, and the three nations essentially encouraged reconstruction within Western Germany by 1947.
To Stalin, and the USSR, the relaxation of economic sanctions and new efforts to encourage reconstruction were potentially not seen as driven by humanitarian concerns, but instead by strategic ones. Stalin, ever paranoid could see in American and British attitudes a distinct desire to rebuild and rearm Western Germany. Considering that the Soviet Union had just endured and fought the brunt of the largest conflict in History, one that had been mostly fought within Europe on Soviet territory, Stalin's misgivings are not entirely unreasonable. Arguably, the institution of the blockade, and the increasing tensions between the East and West are both the result of a Soviet desire to actually prevent further conflict (counter-intuitive though it may sound) the remainder of the Allies could not hope to defeat the USSR, but the allies with the assistance of a newly rebuilt, rearmed and revitalised West Germany would be an entirely different prospect.
Sources: Primary
Foreign relations of the United States. Conference at Quebec, 1944, http://digital.library.wisc.edu/1711.dl/FRUS.FRUS1944
United States Department of State, Foreign relations of the United States diplomatic papers, The Conferences at Cairo and Tehran, 1943, http://digital.library.wisc.edu/1711.dl/FRUS.FRUS1943CairoTehran
Sources: Secondary
Anne Deighton, The Impossible Peace: Britain, the Division of Germany and the Origins of the Cold War
W. R. Smyser, From Yalta to Berlin, The Cold War Struggle over Germany
-edit- This exploded somewhat more than I thought.
I'd like to point out, that whether the East or the West would emerge the victor from another conflict is not the point, more that they simply feared another conflict, and the West being in a stronger position, with strong allies were they did not fear the power of the USSR might consider starting a new conflict to remove communism.
Also, I'm not apologising for Stalin's actions, merely trying to explain why he may have thought they were correct.
I also want to point out that /u/BigBadSaint answers the full question rather than just one part of it as I do.
Because it was on balance, a very soft target.
Part 1 - Background
The events leading up to the blockade was a mix of Stalin feeling that
a) That Germany was being rebuilt into a Western proxy (Bizonia)
b) This impending German union was undermining his control of East Germany (through currency independence)
c) That on a wider scale, the uneasy WW2 alliance was breaking down (cf disagreements over Iran etc)
This is perhaps stating the obvious, but look at Berlin on a map. It is wholly ensconced within East Germany. In the event of war then, there is no way it could be defended by the US. It was therefore a strategic liability for the US - something Stalin calculated the US would rather give up now quietly rather than later, and whose vulnerability could be squeezed through a passive-aggressive blockade . Not only this, but Berlin wasn't actually that significant a prize at the time - sure, it would be a blow to lose it, but the city only became invested with symbolism by Cold War flashpoints like the airlift, JFK's speech, the Berlin Wall etc, so it was something Stalin thought the US could afford to give up.
So far, so conventional.
What gets lost in the mainstream memory of the Blockade is that Stalin actually calculated correctly. The US Joint Chiefs of Staff under Omar Bradley, were strongly advising Truman to arrange for the quiet transfer of Berlin, setting out in detail how the city's vulnerability meant that, militarily and politically (from the POV of the Occupation) it did not make sense to try to keep it.
Truman wouldn't hear of it. There is some great literature in Foreign Policy Analysis on cognitive schemas in decision-making, and a particularly good article on the Blockade by Deborah Welch Larson. Essentially, not really understanding the situation's nuances, and not trusting his advisors, At Lucius Clay's bequest, Truman made a bizarre gut-decision to hold onto Berlin at all costs. The JCS pressured him t either give it up or to grant them permission and funding to plan for all out war, something Truman couldn't comprehend and which clashed with his self-image as a fiscal hawk dedicated to cutting defence spending.
So throughout the crisis, the President really didn't have a policy. Truman's stance was essentially "We HAVE to keep it, but I can't bear to think about how we can keep it'.
Part 2 - The Airlift
The airlift gets remembered now as a strategically brilliant parry to Soviet passive-aggression which left Stalin unable to respond.
Stalin didn't see it coming, that much is true, But that is because it was an incredibly foolhardy decision by Truman.
Firstly, it was lucky. Stalin's initial reaction to the airlift was to laugh and wait for General Winter to arrive and make airlifting impractical and dangerous. As it happened, the airlift occurred through one of the mildest winters in memory.
Secondly, and much more seriously, the airlift represented a dangerous concentration of US and British strategic airfleet in West Germany. To keep the airlift going at the intensity needed to feed such a large city meant basing a large chunk of the airfleet in Occupied West Germany. I don't have a source to hand but IIR it was up to half of the Allied strategic air power.
In the event of a war and a rapid land invasion, Soviet tanks and mechanised infantry would have been able to race across the border and knock out the Allied strategic bombing capability. Not good when the whole US strategy for deterring the USSR depended on strategic bombing
EDIT - as pointed out in comments, the airlift involved mainly C-54 transport planes, not bombers. It is clear that the Air Force felt these to be an important part of the US' strategic capabilities, but not necessarily its bombing capability.
It's like in chess, when an experienced tournament player is playing against a novice , the novice will once in a blue moon make a really strange move ignoring the obvious trap in front of them, but which makes the experienced player momentarily unsure as to what to do against such a weird move? Truman in Berlin was that novice player, and Stalin was the experienced pro momentarily thrown off his game.
Part 3 - the prospects for war
You rightly point out in your question that the US had nuclear supremacy at this time, and Stalin did not want war. However, blockading Berlin rather than attacking it was a very controlled type of escalation - the Allied public were also exhausted of war, and it would have been very hard to justify aggression because a rival country simply wasn't playing ball anymore.
The interesting thing to note though, is that US nuclear supremacy was far from overwhelming. The US had bombs, and the Air Force to establish air supremacy and use those bombs at leisure, but it is doubtful whether they had enough, and within range to actually deploy quickly in the event of a war. Whether Stalin knew the limits of US air and nuclear power is unclear. The efficiency of strategic bombing as a military strategy was certainly very dubious, and was largely the result of a myth created by an alliance of the Air Force with Taft Republicans in the Senate (who liked strategic bombing because it was cheaper than the actual policies the JCS wanted and needed - an expanded draft and universal military training).
Combined with the aformentioned concentration of the Air Force in vulnerable positions, if there was ever a moment in the Cold War that the USSR could have overrun Western Europe, this was probably it: US defence cuts, overwhelming numerical superiority and a strategic blunder by Truman mean that the Berlin Crisis was actually a very precarious moment in retrospect.
But as we know now, the airlift succeeded against all improbability, Stalin's caution got the better of him, and some deft negotiation by Dean Acheson let Stalin host a conference on 4 power rule in order to save face.
Sources
John Gaddis - We Now Know
Melvyn Leffler - The Emergence of an American Grand Strategy
Deborah Welch Larson - The Origins of Commitment: Truman and West Berlin
Follow-up question : One thing I've heard is that Stalin was intentionally creating a conflict in Berlin to divide American attention away from the Chinese Revolution. The USSR had some fear of the US using nuclear weapons to support the Nationalists, so by threatening war in Europe they made the US less willing to use their very limited nuclear stockpile in China.
Is there any truth to this?
I can't talk too much to Stalin's motivations for the blockade, however, his belief in the effectiveness of nuclear weapons was not as encompassing as other Soviet leaders. Keep in mind that during this time nuclear strategy and technology was still in its infantile stage and the classic image of an aggressive nuclear USSR was not true. While after the dropping of bombs on Hiroshima and Nagasaki accelerated Soviet research into nuclear arms, Stalin did not put much weight into nuclear arms.
Soviet nuclear research would not be given much primacy until after Stalin's death in 1953 when military professionals gained more say in the development of nuclear technology (although still controlled by the ruling leader). This change would lead to a massive shift in the 50's and 60's that would lead to the concept of Mutually Assured Destruction (MAD). In 1948, while the US had a monopoly of nuclear weapons, they could not completely dismember the Soviet war machine with their stockpile.
This fact encouraged Stalin, who was influenced greatly by his experiences in the Russian Civil War and the World War II. Drawing from these experiences, Stalin derived Soviet military doctrine from large scale conventional forces and strategies that led to the defeat of Nazi Germany, the largest military threat the world had recently seen.
When it came to defense, the Soviet Union preferred to use politics and diplomacy (backed by a large conventional army) to prevent war and extend power. The Soviet Union used these means for several reasons. First of all, the technological setbacks that plagued the Soviet nuclear program during the late 40's and early 50's meant that the USSR had little hope of matching the US and its Western allies in nuclear arms. Therefore, Stalin downplayed the use of nuclear weapons. He claimed instead that nuclear weapons would not be sufficient for winning a war, and could only be used to frighten another country into submission. Stalin's disinterest in nuclear arms and the overall Soviet belief in political and diplomatic means to prevent war caused the lack of progress in nuclear technology, and to revert to what they knew to work - conventional military force. If Stalin could use Russia's great industrial resources and manpower to defeat the Nazi's, it could be used to defeat the West, especially with nuclear stockpiles too low to constitute a decisive military defeat if used against the USSR.
Sources:
Garthoff, Raymond L. Deterrence and the Revolution in Soviet Military Doctrine. Washington: The Brookings Institution, 1990.
Garthoff, Raymond L. Soviet Military Doctrine. Glencoe: Free Press, 1953.
Garthoff, Raymond L. Soviet Strategy in the Nuclear Age. New York: Frederick A. PRaeger, 1958.
Glantz, David M. Soviet Military Operational Art: In Pursuit of Deep Battle. Portland: Frank Cass, 1991.
Holloway, David. The Soviet Union and the Arms Race. New Haven: Yale University Press, 1983.
Lambeth, Benjamin S. How to Think About Soviet Military Doctrine. Santa Monica: RAND Corporation, 1978, http://www.rand.org/pubs/papers/P5939.html
The Berlin Blockade was a Soviet response to the Marshall Plan and perceived Western separatist policies in American-controlled West Berlin. Upon discovering that the Plan would require economic cooperation, Stalin rejected it and actively attempted to subvert it right from the Paris talks (this is confirmed by the diary of French president Vincent Auriol, and Molotov completely rejected it). The Soviets viewed the Marshall Plan as an attempt by the Americans to gain influence and perhaps even control over countries accepting payment. Andrei Vyshinsky openly stated this in a speech to the UN General Assembly in September 1947:
The so-called Truman Doctrine and the Marshall Plan are particularly glaring examples of the manner in which the principles of the United Nations are violated, of the way in which the organization is ignored.
As the experience of the past few months has shown, the proclamation of this doctrine meant that the United States government has moved towards a direct renunciation of the principles of international collaboration and concerted action by the great powers and towards attempts to impose its will on other independent states, while at the same time obviously using the economic resources distributed as relief to individual needy nations as an instrument of political pressure. This is clearly proved by the measures taken by the United States government with regard to Greece and Turkey which ignore and bypass the United Nations as well as by the measures proposed under the so-called Marshall Plan in Europe. This policy conflicts sharply with the principle expressed by the General Assembly in its resolution of 11 December 1946, which declares that relief supplies to other countries 'should (...) at no time be used as a political weapon.'
As is now clear, the Marshall Plan constitutes in essence merely a variant of the Truman Doctrine adapted to the conditions of postwar Europe. In bringing forward this plan, the United States government apparently counted on the cooperation of governments of the United Kingdom and France to confront the European countries in need of relief with the necessity of renouncing their inalieable right to dispose of their economic resources and to plan their national economy in their own way. The United States also counted on making all these countries directly dependent on the interests of American monopolies, which are striving to avert the approaching depression by an accelerated export of commodities and capital to Europe (...)
It is becoming more and more evident to everyone that the implementation of the Marshall Plan will mean placing European countries under the economic and political control of the United States and direct interference by the latter in the internal affairs of those countries.
Moreover, this plan is an attempt to split Europe into two camps and, with the help of the United Kingdom and France, to complete the formation of a bloc of several European countries hostile to the interests of the democratic countries of Eastern Europe and most particularly to the interests of the Soviet Union.
An important feature of this plan is the attempt to confront the countries of Eastern Europe with a bloc of Western European states including Western Germany. The intention is to make use of Western Germany and German heavy industry (the Ruhr) as one of the most important economic bases for American expansion in Europe, in disregard of the national interests of the countries which suffered from German aggression.
The "measures taken by the United States government with regard to Greece and Turkey" that he is referring to is the famous Truman Doctrine, announced in a speech on March 12, 1947, prior to the Berlin Blockade and foreshadowing the events to come. It is commonly cited as the start of the Cold War.
So while many say that following the end of the War the USSR was aggressively expanding and that the Berlin Blockade was part of this; from here it is clear that the Soviets perceived the Marshall Plan as aggressive American expansionism into Western Europe. The Berlin Blockade was the result of a build up on both sides, and not an act of Soviet aggression against West-German and American victims, as is commonly perceived.
In March 1948 Truman unknowingly summed up the position of both sides when he said that "The situation in the world today is not primarily the result of the natural difficulties which follow a great war. It is chiefly due to the fact that one nation has not only refused to cooperate in the establishment of a just and honorable peace but—even worse—has actively sought to prevent it."
The Marshall Plan ("Economic Cooperation Act") was signed into law on April 3, 1948, which is around the time that the events surrounding the Berlin Blockade started. The Blockade itself officially commenced shortly thereafter.
Follow-up question --
A lot of this is getting into alternate history speculation, which I think is a fool's errand, but what discussion there is seems to me to be overestimating the strength of the U.S. nuclear arsenal by that time. Are there reliable sources on the size of it?
For reference, Google gives me one, which claims to be quoting a 1983 Department of Energy report titled The United States Nuclear Weapon Program: A Summary History. It says there were all of 13 functional nuclear weapons at that point, which seems to me a limited supply to start using them against massed armor formations in the field.
Because he believed that the U.S. were anxious to get out of Europe and Britain lacked the capacity to stand up to them. Churchill had wanted the U.S. and Britain to limit Soviet expansion into Europe but couldn't persuade Roosevelt or Truman. Stalin thought if he pushed it the U.S. Would think it was too much trouble and go home.