In Embracing Defeat John Dower does a great job explaining how Japan was rebuilt following WWII. While the book is mostly focused on the effect of the war in the Japanese psyche, he also talks about American ambitions. Essentially, Japan was needed as a curb against the growing communism in China and the USSR.

China needed to focus on it's own rebuilding efforts, as well as worry about the other China (Taiwan). Likewise Europe was worried about Europe (although Britain did contribute a little to the rebuilding efforts in Japan.) The Soviet union was never significantly invested in the Pacific theater and had China as a communist ally in the region. Basically the United States was the only state with both the capacity and interest to undertake rebuilding Japan.

It's worth noting though that Japan's colonial holdings were divided amongst the great powers. North Korea to the Soviet union, Manchuria Back to China, Hong Kong to the UK.

It did. To explain, first you have to understand what happened in Europe. The 4 allied nations of USA, USSR, UK and France made plans before the war's end on how to administer the defeated nations. They would divide them up into zones, with each country having a zone they controlled, and they'd begin the process of recovery and establishing viable governments and economies in the defeated nations. Plus major cities (Berlin, Vienna) would be divided into 4 zones themselves, so even though the USSR controlled all the ground around Berlin, a portion of Berlin was controlled by the US, France, and UK, too. The US, France, and UK established governments friendly to themselves and capitalist economies and democratic governments in the areas they controlled. The USSR set up puppet communist states out of all of the eastern countries under their administrations, even making some, like Latvia, into parts of the USSR. Tensions heated up between the two factions and Winston Churchill coined the term "iron curtain" to define the figurative wall separating the west from the soviet-controlled eastern bloc countries. Since both the US/UK/France controlled part of Germany, and the Soviets controlled another part, they couldn't agree on a single state, and two states were set up. Berlin was deep in the Soviet controlled East Germany, but the US wouldn't cede control of it to the Soviets, so we ended up with East Berlin from the Soviet controlled area, and West Berlin from the US/UK/France controlled area.

OK, so what happened in Asia? The Soviets declared war on Japan only a couple days before the Japanese surrendered. They knew Japan was surrendering within days so that's why they declared war--they had a mad land grab. They seized the Sakhalin islands north of Japan that were controlled by the Japanese, and they seized the northern part of Korea--at that time, Korea was part of Japan. Japan had annexed it in 1905 and Korea lost its status as a sovereign nation though they yearned for independence. Then, the US almost alone took control of the post-war Japanese occupation. The Soviets kept the Sakhalin Islands. The Americans took away from Japan the lands that had been under Japanese control but that were not originally Japanese, such as Taiwan and the Philippines, and set up new governments. Okinawa was also one of the areas seized by the Japanese. The USA kept Okinawa until 1972--it was not part of Japan nor was it its own nation. In 1972 the US released it to determine its own future, and they decided to be part of Japan, so now they are. The US and the Soviet Union each administered the part of Korea that they controlled. But there was no agreement between them about what to do about Korea, since both the US and USSR wanted to set up their own puppet government. In 1950, the Soviet and Communist China-backed Korean area, led by Kim Il-Sung (who had spent much of World War II living in the USSR), attacked the US forces in the part of Korea they administered, and the Korean War began. That's why there are 2 Koreas. So Korea, which had been Japan from 1905 to 1945, was split between the US and USSR, much like Germany had been. The rest of Japan was kept in US hands, since the Soviets had never occupied any of it. The Soviets held what they'd grabbed in those last 2 days of the war only. I'm sure the Soviets would have been delighted to have more of Japan, but they just never got there. And the US was the only nation with nuclear weapons at that point, so the USSR let it be. They only tried to grab the rest of Korea after they themselves had nuclear weapons, too.

I think Japan lost the Kuril Islands to USSR, so in a sense it was split between the two powers, even if it's just a small piece.

It's also significant that the US and Japan were at war for nearly 4 years. The USSR only declared war against Japan in the final days, and as noted elsewhere, was only involved in taking land that was on mainland Asia (hence Manchuria getting returned to China and Korea getting split).