After WWII, three important things happened:
- Japan's military forces were disbanded
- A revised Japanese Constitution was drafted which included Article 9, forbidding the establishment of a military body for the purpose of waging war on another country.
- As part of the post-war rebuilding, Japan became an occupied territory under the control of the USA.
The third item is particularly interesting, not only did US forces occupy Japan, they carried out a systematic rebuilding and revitalising process, helping the country recover from the war and end up coming out at the end in a strong condition. Since item 1 meant Japan (like the other Axis powers) had no capacity to defend itself, the victorious states agreed to protect them from aggression. Thus the US-Japan relationship became very strong over time, it's probably one of the better examples of two bitter enemies setting aside their past and working together for a better future.
It's not exactly fair to suggest Japan is defenceless though, as pacifism suggests. Article 9 is generally interpreted that Japan is forbidden from initiating a conflict, but it is allowed to engage in self-defence. To that end Japan does have a military body, the mentioned JSDF, which is actually quite capable and has strong backing from the US throughout its development.
Your mention of Gulf War II is a little off base. They weren't unarmed, but rather given instructions to only open fire if fired upon. Additionally, their protection was partially provided by the US trained Japanese Special Forces. There presence was for reconstruction purposes, so they didn't really have much need to engage in offensive action.
Of the countries in the Pacific and East Asian theatre, Japan and South Korea are the two with the strongest ties to the US government. Since South Korea is a terrible place to build a peace keeping force, Japan is the obvious choice for a primary ally in the area. Particularly since the mutual defence treaties are still in place, giving NATO permission to violently oppose any military actions by NK, Russia or China that are considered to be a declaration of war against Japan.