The alliance with Japan works both ways. It is important to them too, and that said, they would cooperate with diplomatic measures to resolve any issues.
Iraq poses a threat to its own people and its neighbors, as well as Israel. Military action IS a last resort, people just don't seem to realize that with Iraq, after 12 years of attempted diplomacy, we have indeed come to the last resort.
Saddam's regime is executing POWs. Any regime who would carry out such acts is clearly not otherwise peaceful.