Political motives behind US operations
The United States began flexing its military muscles in the South China Sea last year as part of its political and diplomatic maneuvers in the region. The US knows better than any other country that it cannot directly prevent China's sovereign activities in the South China Sea. So it has attempted to do the next best thing, that is, make it more costly for China to safeguard its sovereignty through military, political, diplomatic and media tools. It is thus clear that the US' aim is to put China in an embarrassing diplomatic position.
Despite the China-US friction in the South China Sea, however, a direct conflict between the two countries is not imminent. The US military's moves in the South China Sea may apparently be aimed at deterring China from expanding its influence, but they are primarily meant to serve as tools for accomplishing Washington's political and diplomatic goals.
On "freedom of navigation operations" in the South China Sea, the US has always said they are no different from such practices in other parts of the world. Some Chinese media outlets and experts seemed to have bought the US argument and said the US' aim is to challenge China's "excessive maritime claims".