In what amounts to a Freedom of Navigation Operation (FONOP) Japan will sail 2 of its warships and a submarine from Subic Bay, Philippines (see map above) to Cam Ranh Bay, Vietnam. This will cross China's "nine dash line" (see map below) the vast sea-space (most of the South China Sea) that China claims.
Pete's summary - These newspaper and news agency reports have likely come from Japanese military sources who say Japan will be sending an (Oyashio class?) training submarine and 2 destroyers(?) to visit the Philippines. They are likely to set out in late March 2016 on Japan’s normal patrol route of the Bashi Channel (Luzon Strait) and reach Subic Bay (now a civilian port) in the Philippines in early April 2016. They will then sail across the South China Sea to Vietnam’s largest naval base at Cam Ranh Bay. This will be the first Japanese submarine visit to the Philippines in 15 years
“It needs pointing out that cooperation among relevant countries should be conducive to regional peace and stability instead of targeting a third party, still less harming the sovereignty and security interests of other countries. Japan once illegally occupied China's islands in the South China Sea during WWII. We are on high alert against Japan's attempt to return to the South China Sea through military means.”
JAPAN'S SURVEILLANCE AID STRATEGY
Japanese Defense Minister Nakatani is traveling to the Philippines in April 2016 to discuss cooperation. This includes providing the Philippines with several used Beechcraft TC-90 King Air for surveillance of the South China Sea. Japan and Vietnam have also discussed holding a joint naval exercise
REGULAR US SUBMARINE AND SHIP VISITS
The US Navy has been doing fairly regular submarine and ship visits to Subic Bay, Philippines. On March 9, 2016 GMA News Online reported that US Los Angeles class submarine, USS Charlotte (SSN-766), moored at Subic Bay on March 9, 2016. US cruiser, USS Antietam, docked in Manila Bay on March 8, 2016 and 7th Fleet command ship USS Blue Ridge, docked at Subic Bay on March 4, 2016.