Its claims are in conflict with the claims of smaller nations of Southeast Asia, which, so far as I can tell, are equally valid in international law.
The Obama administration is preparing to confront China militarily over these claims. This is a big mistake.
The sea routes in the South China Sea are vital to China and not vital to any other nation. The South China Sea route is the cheapest and most convenient sea route for Japan, Korea and the nations of Southeast Asia. But if worst comes to worst, they could take a longer route. The Pacific Ocean is a big body of water.
The United States government is currently confronting Russia and China, the only two nations in the world that are beyond the reach of American naval and air power, over matters that the Russian and Chinese governments see as vital to national survival, and which are not vital to the United States.
In the case of Russia, it is the goal of bringing Ukraine into an anti-Russian military alliance and making Crimea a possible base for NATO forces. In the case of China, it is the goal of U.S. domination of the sea routes to eastern Asia.
I am not an admirer of the Russian or Chinese governments. They both abuse human rights. They both believe in their own versions of exceptionalism, believing they have the right to dominate their smaller and weaker neighbors. An increase in Russian or Chinese power is a bad thing, not a good thing.
But I don’t think trying to roll back the existing Russian or Chinese spheres of influence is worth risking war over, any more than Russia or China would think it worthwhile to risk war over U.S. domination of the Caribbean and Central America.