Until the OP gets around to answering...

A little of both. In the first place, Stalin was paranoid, and out to get any one who might conceivably be a threat to him, including every competent individual in a leadership position in his own state. In the second, Hitler had been beating an anti-Bolshevik drum from the outset of his political career. His system of alliances was known as the Anti-Commintern Pact; we just call them the Axis because we didn't want to remind people we were fighting for the survival of communism, and words with higher Scrabble scores, like "axis", sound more villainous. Both Hitler and Stalin went into their alliance with the same cynical mindset that the Hunger Games tributes from the richer districts used - get rid of everyone else first, then settle up between ourselves. Stalin was essentially buying breathing space and getting Hitler to fight their mutual enemies while he built up his buffer zone and restored the Soviet Russian Empire to its old frontiers to be better prepared for their eventual conflict, while Hitler was seeking to outweigh the Western pressure on Poland to resist his demands for the return of the Polish Corridor that separated East Prussia from the rest of Germany, and the German city of Danzig. Helping Poland resist Germany effectively depended on the cooperation of the USSR, since the bulk of the largest country outside of Russia was between France & Poland, and by bribing Stalin with territory the West could not then acquiesce to his seizing, he had, to any rational mind, left Poland & the UK with no option other than to make his deal. For both Germany & the USSR, the partition of Poland was an area in which they were both interested, as was a cessation of hostilities while they settled side business.

Both states were set on conquering the other. Prior to the establishment of the USSR, Germany was the country the socialists and communists intended to be the heartland of European communism. Russia simply fell into their laps with the revolution. They immediately tried to exploit the cessation of fighting in WW1 by invading Poland and starting revolutions in Germany & Hungary, among others. Just because there was no government giving them orders though, did not mean the demobilized Germany army forgot how to fight, and they simply teamed up again to protect their country against both communist revolutionaries, and various newly independant nations seeking to bite off chunks of German territory. Hitler capitalized on the successes and achievements of the Freikorps in his political campaigns, and used the memory of the attempted revolutions to gain support, first for his platform, and once in office, for his security measures. Despite being birds of a feather, and kindred spirits, Hitler and Stalin were inevitable enemies, and each one's regime occupied a country which was critical to the other's long-term objectives. Hitler needed territory and natural resources to make Germany a self-sufficient power, and sought to break the old Prussian-German nightmare of strategic encirclement. Stalin sought the elimination of his major rival for power on the continent, and a platform from which to advance world socialism, Russia being considerably behind the West in economic, intellectual, technical and industrial ability and human capital, as well as being geographically isolated.

They weren't. The American people were adamantly opposed to another European war. The perception at the time was that they had been led into the last one with idealistic aspirations, only to lose 100,000 dead, with nothing to show for it but an enlargement of the British Empire, a punitive peace, and repudiated war debts from our Allies. Only Finland, of all the European states, actually paid off its American debts, while American loans were the sine qua non of Allied ability to carry on the war until we joined in. Roosevelt was reelected in 1940 asserting that he would never send our boys to Europe, and while this was not a remotely sincere or honest promise, it pandered to the desires of the electorate. Great Britain was not bearing down on shit. The war that the UK declared on Germany in 1939 was already lost, and the Empire's ability to make war on Germany was effectively nil.

Yes. Not "easily" in the sense of "without a great deal of effort", but easily in the sense of "outcome is not in doubt". While much is made of the production capacity of the USSR and its superior natural resources and manpower, Germany had all but taken those resources by the end of '42, and the population of the Soviet Union was little more than twice that of the Greater German Reich, plus its allies, whose agreement with Germany was aimed specifically against the Soviets. Twice the population sounds like an impressive advantage, until you read the casualty figures, and see that Germany was killing Russians at rates ranging from 3:1 to 5:1 throughout the war. The enumeration of military and materiel losses in the opening days of the war alone were a devastating blow to Soviet military capabilities. While the Soviet propaganda and a supine Western academic community that accepted said propaganda at face value claimed they stood alone against Hitler and that they did the bulk of the fighting, that's like claiming that a man in the forest killed by a wolf was really killed by the dozens of mosquitos that bit him beforehand, because there were more mosquitos attacking, for a longer period of time, and the mosquitos took more casualties than did the wolf.

Setting aside the battlefield contributions of the Western Allies (which basically means America, since Britain and France were in no condition to fight a major war without America propping them up), the Soviet Union was living off of American supplies & materiel. US companies set up production facilities in the USSR and the USA provided thousands of railroad engines and cars, which were critical to Soviet logistical issues. 90% of trucks used by the Red Army came from the US or UK, as did half the tanks. German accounts of the fighting in the East describe many encounters with American-made Sherman tanks. The vaunted T-34 was produced in large numbers...according to Soviet records, which were also largely influenced by Soviet propaganda, when they were kept properly at all. Furthermore, Soviet production figures are complicated by little details like reporting shortfalls and failures to meet quotas was a good way to get shot and your family imprisoned, so production reports tended to be very positive, regardless of the reality. Soviet quality control was practically oxymoronic throughout the history of the state, and shortly before the outbreak of hostilities, a leading official in the air force was shot for reporting a large number of aircraft and pilot losses due to issues in that area. Most Soviet tanks, however functional their designs, were so shoddily made that their operational life was measured in days or hours before needing extensive maintenance, which was beyond the skills of the typical Soviet conscript. The typical Russian tactic of the war was massed frontal assaults, because that was the extent of the abilities of their officers to coordinate. The major Russian defensive victories in the first couple years of the fighting were propaganda. There was no "Battle of Moscow", simply the Germans calling off operations for the winter when they ran into supply and maintenance issues in December. Stalingrad was a clear cut case of the Russians preventing the Germans from taking a major industrial city, but the fighting rendered the city worthless, and it was all the Soviets came away with from a massive series of winter offensives, named after the outer planets, across the length of the front. All along the front, the Germans slaughtered Soviets by the truckload, and largely contained their advances, but the Soviets managed, at great cost, to cut off a ruined city, thanks to the ineptitude of a German general with little to no field command experience, and turn that Pyrrhic victory into a propaganda triumph. The Germans were prosecuting a successful offensive the following summer, when Hitler called it off to redploy his forces elsewhere, and the Soviets simply named the fighting after Kursk, the first town still behind their lines, to give the impression that the battle was intended to take the city, and thus was a German failure, because the Russians retained control of it. It would be like calling Cannae the Battle of Rome, and claiming Hannibal got his ass kicked because he didn't manage to take the city during that battle.

Meanwhile, almost every major Soviet success or advance was made in conjunction with the Western Allies or capitalized on Western actions. They managed to conquer less than half of Poland, by only invading two weeks after the Poles well and truly had it stuck in against Germany. The exaggerated victory of Stalingrad, though small potatoes when set against the concurrent battlefield humiliations elsewhere, was undoubtedly helped by the diversion of elite German units by Western action. The 1st SS Division, one of the most heavily armed units, whose firepower matched most Soviet armies, was held back in the west as a reaction force in response to the Dieppe Raid, which German leadership feared might be the first wave of an Allied invasion. The Afrika Korps under Erwin Rommel was supporting the Italians against the British, and either one of those units added to Manstein's command might have enabled him to link up with the 6th Army cut off in Stalingrad. The redeployment I aluded to above, from Citadel, was a reaction to the Allied invasion of Italy, and the need to protect Southern European strategic assets from the American & English forces. The first major Soviet push out of Russia, was Operation Bagration, which shattered Army Group Center in Belarus...a week after D-Day, which stripped away most of the armor and air support from Army Group Center in order to defend France. And that's in addition to the Allied bombing campaigns, which, while having little real effect on the outcome of the war, still tapped into the stretched thin German forces to defend. Fighter planes and anti-air artillery were required to reap the brutal toll they did, but these assets were not available elsewhere (and the Germans made extensive use of anti-aircraft guns for ground purposes as well; planes and AA guns were often more useful against heavy tanks than tanks themselves). Finally, the US war against Japan freed the USSR from having to worry about their own Eastern Front.

Not particularly relevant to the outcome of the Russo-German conflict, was that the alliance between the US & USSR provided all sorts of opportunities for espionage, infiltration and influence in the subsequent Cold War. The US treasury printing plates, given to the Soviets to enable them to print the joint currency of occupied Germany proved a financial windfall to the USSR, as they could buy enormous quantities of Anglo-American goods with monopoly money, create inflation in the Western occupation zones, and get a jump on counterfitting operations. Lend-lease was used as a cover to obtain all sorts of US technology and even consumer goods which the Soviets could use in building up their infrastructre or reverse engineer to improve their own technology, and a great deal of classified information left the US in Soviet diplomatic pouches during the war.