China might have plenty of reasons to be confident about its expansionist ambitions in the South China Sea, according to a recent US Office of Naval Intelligence (ONI) report that lays out China's long-term strategy for dominating its neighborhood.
China wants to develop a navy that can operate far from the country's coastline, establishing unquestioned military superiority in the Pacific and Indian Oceans and deterring its regional rivals from standing in the way of its expansionist policies in the East and South China Seas. 
China wants to project its military power deep into the claimed territory of other rising regional military powers, at the same time as the US' "pivot to Asia." And it thinks it can grow its strength in a way that won't trigger a violent crisis and actually reduce the chances of war in the long run.
Those may be mutually exclusive objectives. The more a country expands its military footprint, the greater a chance that its military rubs up against the territory, personnel, or national prerogatives of rival powers in ways that nobody can control or predict. 
At the same time, this chart from the ONI report helps explain why China thinks it can deter its neighbors. It just has far more ships and resources than any of its immediate rivals.
Screen Shot 2015 04 17 at 3.01.52 PMUS Office of Naval Intelligence
This chart shows how many "naval law enforcement vessels" various countries in East Asia have at their disposal. This measures Coast Guard-type ships, rather than combat vessels.
But China has largely enforced its various claims in the South China Sea — which overlaps with the exclusive economic zones of Japan, Vietnam, and the Philippines — using its newly reconstituted coast guard. It has over twice as many enforcement ships at Japan and Vietnam, and that's before even counting vessels specifically built for combat. China has a large enough contingent to be able to operate just about everywhere that it is contesting territory, while still having scores of ships in reserve.
China is counting on being too large and powerful for its rivals to be willing to take on. Whether they're right in the long run, this chart shows that they at least have a major quantitative advantage.

Read the rest of the ONI report here.