It may be a "boiling frog" issue and it is certainly complex, as indicated by David. However, this does provide an opening for the US. Our pursuit of the "war on terror" has led us to take our eye of certain aspects of the Pacific.

The main US interest here is maintaining freedom of navigation in the commons. This interest conflicts with China's traditional view on sovereignty, which takes on a nationalistic tone given their history, specifically what they call "the century of shame and humiliation," which refers to their exploitation by western powers. If I was in their shoes, I'd probably take the same position. The problem here is that their position is contrary to international law.

I'm no expert on law of the sea, but since they are signatories to the law of the sea convention they are bound by its provisions and it repudiates their claims. Believe it or not, international law can be just as much of an achilles heel to them as it can sometimes be to us. This is where we can press them and use the issue to bring other countries in the region closer to us. This should be framed as an economic, trade issue rather than a security issue though. The last time a rising Asian power headed south for economic reasons, Pearl Harbor was attacked. It is a security issue for us, but I think we get more play using law and economics as weapons of choice.

By the way, this whole maritime issue also plays into China's actions in space. The goal is access denial by continually pushing the boundaries of sovereignty. Whereas Europeans and some others are chipping away at sovereignty, China embraces it and seeks to expand it in novel ways. The lawyer in me likes the strategy even if I do not agree with the tactics.