The PRC supported Kim Il Sung in 1950 and they will support Kim Jong Il if the Korean War starts.

Both the PRC and the DPRK are totalitarian states which do not tolerate dissent.  They support each other
politically and economically to provide the wherewithal for aggression, be it against the RoK and Japan or by taking over key shipping lanes in the South China Sea.

Additionally, the PRC/DPRK share a racist mythology which identifies Chinese/Koreans as being a master race.
It's not by accident that DPRK troops goose step during parade marches.

Hence it is worrying that the PRC/DPRK have weapon systems which would likely defeat our surface fleets as as well as our anti-missile defenses.  This makes the US unacceptably vulnerable to attacks by enemies which would like to annihilate us.

The PRC has cleverly lulled the West into complacency by paying lip service to issues such as the Paris Accord which makes them popular albeit they are carrying out aggression which threatens the West.