Fascinating discussion, with some interesting points. A couple of thoughts:
1) The outcome of the war wasn't a foregone conclusion at its start--it only was from about 1943 on. But the Axis powers didn't expect to fight a world war--they thought they could grab some territory and end it. Think about the Japanese experience in the 19th and 20th century. In the 1860's, Japan is arguably the most advanced civilization in the world, but completely isolationist. The U.S. forces open their borders with a quick show of military force. The Euopeans show up in China and India, make some quick victories, and divide the land up. The U.S. has a couple of quick, decisive victories over Spain and attains all their far eastern possessions. Japan wins a few quick decisive victorys over Russia and acquires territory. Japan declares war on Germany during WWI and snatches some territory. The modus operandus of nations throughout this time period, especially in the far east, was to win a few decisive victories and force the other side to negotiate a quick peace--the myth of the "short, decisive war". So WWII rolls around, and Japan decides to win a few, quick decisive victories and grab some territory. What do the American people care if Japan, not the U.S., imperialistically dominates the Phillippines? Japan understimated the resolve of the Americans to pursue the war to the end. Even at the end, before the A-bomb, their thinking was that if they could bloody the Americans' nose enough, the U.S. would accept a conditional surrender and sign a peace treaty.
Germany believed a similar thing, and why not? They had grabbed Austria, and Europe had watched. They had grabed Czecheslovakia, and Europe had watched. They had grabbed Poland, and started the war, but even after the formal declaration, France and Britain sat and watched. And why should Britain care if Germany attacked the Russians, I'm sure Hitler thought. After all, the British feared communism as much, if not more, than Nazisim. Again, there was an underestimation of the resolve of the allies. And this resolve may almost have not been realized.
Roosevelt was facing serious opposition to continuing Lend-Lease, let alone entering the war. If Moscow had fallen to the Germans in 1941, or if Japan had not attacked the U.S. and Russia had fallen in 1942, the war might have ended with a German superpower, not a Russian one.
2). Regarding the idea that we could, and should, have just "taken out the Russians when we had the chance"--well, that is exactly the same mistake that Germany and Japan made at the start of the war. As we're finding out in Iraq, there is no such thing anymore as a "short, decisive war". Maybe we could have had a quick MILITARY victory over the Soviets in 1946 (although not necessarily---(1) the Soviet troops, thought bloodied, were in a high state of readiness, with a great deal of combat experience, and still had strong defensive infrastructure over the long length between Berlin and Moscow. (2) The Soviets had a LOT of land, and the U.S. had essentially only 1 A-bomb left, and A-bombs could only be built maybe one or two a year. Yes we had the B-29, but destroying cities does not equal conquering territory. We would have had to move troops all across the Atlantic and Europe to even get to the front, and then fight a determined foe, when the American people were all ready war-weary and wanting the troops to come home). However, even if we did triumph militarily, the amount of troops necessary to hold Russia and REBUILD it into a nation that is favorable to us would have been far beyond the capacity of the U.S. and Britain in 1946. I mean, we can't even do that in Iraq today, and Iraq is a lot smaller than Russia. Militarily defeating the Soviets would have left the country in chaos, we would have been unable to rebuild it in our image, and eventually, some new power would have arisen in Russia with an even stronger fear, distrust, hate of the U.S. and western Europe.