Air Force Secretary Frank Kendall on Monday warned that China was building up its military to prepare for a potential war with the U.S., and he said America must optimize its forces to counter the rising threat.

Speaking at the Air and Space Forces Association Warfighter Symposium at National Harbor, Md., Kendall said the U.S. must be ready for a “kind of war we have no modern experience with,” though he stressed “war is not inevitable.”