We do not underestimate China. In fact, we usually overestimate any potential adversary, if only to get more defense funding according to some small minded people here.
The fact that China have no military adventures compares to US is actually a disadvantage for China, especially when it comes to technologically laden type of warfare like air and sea. On land, troops can disperse and regroup. At sea, that is not possible.
World War II was the first and likely the last time that fleets will face each other in combat. Air power made that possible. All attacks will be the over the horizon type and in this, the US have the gross advantage -- the aircraft carrier. In the SCS, PLAN ships will be vulnerable to the nth degree, so will be those stations on those man-made islands. All it will take for China to learn not to engage the US Navy is the loss of just one PLAN ship. China will withdraw.
