What's happening in the SCS is part of a struggle between US/allies to maintain American primacy as basis of the Asian security order, and China's aim to change it. In other words, great power geopolitics have returned from 1989 with gusto, and currently focused in the Asian maritime periphery. One could demonstrate US policy towards China shifted from cooperative to competitive in Bush43's administration, and from competitive to proto-adversarial in Obama's administration. The proof is Bush said on national TV that China was a "competitor," and Obama said US dealings in Asia was about who leads the region. If we take US leaders at face value, then US policy evolved from cooperative to hedging, then containment, and now we have encirclement.
Rational people must see the world as it really is and not how they want it to be, right or wrong has no bearing on that effort. From that perspective, the evidence is overwhelming US has indeed taken sides, and it will do whatever in its power to contain and even encircle China. On the other hand, China will do whatever is in its power to break containment or encirclement and rearrange the Asian security order.