Be honest, can war still be instigated against any developed nation? The answer is no. There are obvious reasons, but I will be focusing on the economic, political, military, as well as the brutally honest side of things.
The developed world are called that for a reason, and the main point is they control the world of economics, being allies with each other and having multiple treaties with each other means no nation as of yet can challenge them. China may look big, but relative to G6, we are but a fraction of what they are combined, or what the Us or EU is alone.
Taking the war to them would mean economic sanctions, and even if it doesn't come to that, trade would be greatly disrupted, and no developing nation can separate from the developed world and land softly.
Politically, the G6 nations still control all of the international institutions and a declaration on one, is essentially a declaration on all. The other important point is the developed nations all share a common ideology and has the resources available to promote human rights rather than just preach it. This makes any nation declaring on them, strong or weak, seem evil, this will be expanded on later.
Militarily, the US is easily the greatest fighting force the world has ever seen, and Japan + EU are powers in their own right. While Japan may still have a peaceful constitution, that constitution clearly has not reached their navy, as they boast, easily, a top 5 navy in the world, what they lack in air force and ground force are easily made up by the American presence in Korea and Japan.
The G6's combined military spending hovers around a trillion dollars a year and can dwarf any nation that even thinks of matching them.
Lastly we get to the part where no one wants to talk about, the developed world has people that are just worth more. They have a voice. Their media dominates the global narrative and their reputation makes sure their words are taken at face value by most uninterested viewers, and hard to argue, by the hardcore followers.
The hurricane Katrina, Japanese nuclear disaster, French shooting, and more are terrible events, but hardly the worst that have happened that year, or even that week in the world. Yet, nobody knows or cares about the bloody battles that goes on in the rest of the world, be it civil wars, terrorism, or war on drugs.
So the question remains, can any nation hope to declare on a developed nation today? The Americans went into Iraq with lies, but the idea of freedom, and democracy as well as prosperity appeal to the masses, and thus, condemned, America more or less still faces little backlash over its action, while on the other side of the pond, China faces severe backlash, and even given the Nazi tag, over disputed territories and no military actions of any kind.
Bottom line, China and Japan will not go to war, it doesn't matter if Japan has 0 or 20 carrier groups.