By now it is quite clear that the US government made the decision of using atomic bombs on Hiroshima and Nagasaki as a way of claiming Japan as its sphere of influence (before the Soviets got into action) and also probably as a way of simply demonstrating the newly enhanced imperial might to the rest of the world, not just to the Soviets. The Smithonian exhibition that had originally attempted to place the use of the atomic bombs in the context of US anticommunist policy got quickly censored, as you may recall. For this part of US history, read, for instance, Stephen Shalom and Gar Alperovitz.
As to American help to 'install democratic systems,' little did Japanese lovers of democracy suspect that it would come with red purges and re-installment of war criminals and keiretsu in power, but they were naive. ('Denazification' in West Germany was a big joke as well. Films such as _The Nasty Girl_ and _Zentropa_ are pretty clever critiques of this farce.) On the other hand, Americans did introduce land reforms, which I think had an effect of creating enough smallholders to serve as one of the main constituencies for the Liberal Democratic Party, shoring up a popular base for capitalism.