The limits are usually set by international conventions and agreed boundaries. The concern is that China does not recognise either. Their arbitrary "9 dash line" and the building up of reefs and shoals into islands have already gone beyond international limits. They don't see the taking of Taiwan by force as a limitation but the West does (presumably) does. So the limits are dependent on your point of view and ability to enforce them. Its normally resolved through diplomacy but when that fails its called war. In my view the parallels to the 30's will continue all the way to 1/9/39.