I believe their intention is to take action without China stopping it. They know China has been rattling sabers with Japan for a while over some disputed islands. They probably believe China won't stop them if they go after Japan but will interfere if they go after SK, Taiwan, or the US as China needs stability to fight on the economic front.
Your thesis has merit. Those disputed islands are the Spratly islands I believe and I think Vietnam is a bit more involved in this controversy as is the Phillipeans. Of course the issue is potential oil resources.
I came to that conclusion as well. China will let them go after Japan.