The nine-dash line has allowed China to claim a right to do things ranging from drilling for oil to actually creating man-made islands — without providing anything specific that other nations could argue against. Tuesday's arbitration ruling could make it increasingly difficult for Beijing to play that game.
According to one theory, China's South China Sea island-building and its deployment of naval and air power in the area could signal that it is hoping to turn the entire zone into a Beijing-controlled "strategic strait."
The Philippines claims that direct negotiations with China proved to be a dead end — and the Philippines certainly can't persuade China with military might. Similarly, the other nations in the region have made little headway with China concerning their own competing South China Sea claims. Beijing insists on one-on-one negotiations, and no one can stand up to China by themselves.
An international tribunal ruling against the nine-dash line would go a long way toward offering a framework for a unified front against China, and that is something that worries Beijing, experts said. Such a decision could "give more hope to the Philippines and other Asian countries that claim territory in the South China Sea," according to Andrew Scobell, a political scientist at the Rand Corp.
Beijing fears a blow to its global reputation if it is seen as flouting international law, but on the other hand the Chinese Communist Party would have a hard time explaining any acquiescence to a Chinese population that's been drummed into a nationalistic fervor over the issue.
"The power of international law is primarily reputational and measured in terms of legitimacy," Mira Rapp-Hooper, a senior fellow in the Asia-Pacific Security Program at the Center for New American Security, told CNBC earlier this year, when China's island-building in the region came under scrutiny. "My speculation would be that China has basically calculated that it will take some near-term, rather assertive actions in the South China Sea, and pay short-term reputation costs in exchange for what it believes to be longer-term strategic gains."
Many Chinese, politicians and regular citizens alike, see opposition to their South China Sea claims as opposition to their ascendance on the world stage.
"China is a rising power and it is feeling restrained by U.S. military presence in the Western Pacific," said James Keith, former China director for the National Security Council. "China is fighting back against American dominance as it tries to carve out a place for itself in the region."