Of course the IMT was a joint effort, it was unthinkable not to be; still, the initiative was mostly American (and from Soviets, to be honest, always sensitive to a bit of good propaganda). Churchill (with other "traditionalists") was much more inclined to old-school measures.

It turns out that the IMT was a fantastic idea, probably a bit too successful -- it made more difficult for nation states to (ab)use their military forces, at least until the end of the Cold War.