Seventy years after its Imperial forces were kicked out of the South China Sea, Japan is quietly moving back into the region, forging security ties with the Philippines and Vietnam as both Southeast Asian nations try to cope with China's territorial ambitions.

Tokyo's security cooperation is broad-based: It is supplying maritime patrol boats to the two countries and will hold its first naval exercises with the Philippines in the coming months. Japanese military doctors are even advising Vietnamese submariners on how to deal with decompression sickness.

Japan is providing this help, and more, in a calibrated escalation of involvement to avoid a backlash from Beijing, said Japanese sources with knowledge of the assistance.