I think they are tied together. China and the US are the two biggest world powers. Any conflict between them would draw Japan in. Japan also has rivalries with China and North Korea. Any conflict between Japan and one or more of those would draw the US in.
If the there is a conflict in the eastern Pacific, Australia would like join in as well. It is also highly likely that the UK, Netherlands, Denmark, Poland will likely join as well and possibly Germany and France.
I mean if you're considering his reign a "peaceful time" you're missing all the things that have happened in Africa, the Middle Ease, or Latin America. I think we just have different definitions of "peace" here. Sure, nobody is dropping nukes or waging a world war, but there's still been constant conflicts somewhere.
If the there is a conflict in the eastern Pacific, Australia would like join in as well. It is also highly likely that the UK, Netherlands, Denmark, Poland will likely join as well and possibly Germany and France.