India hosted American and Japanese aviation ships in naval exercises in the Indian Ocean:
"Malabar 2017 is the latest in a continuing series of exercises that has grown in scope and complexity over the years to address the variety of shared threats to maritime security in the Indo-Asia Pacific," the U.S. Pacific command said.
Military officials say the drills involving the U.S. carrier USS Nimitz, India's lone carrier Vikramaditya and Japan's biggest warship, the helicopter carrier Izumo, are aimed at helping to maintain a balance of power in the Asia-Pacific against the rising weight of China.
All three countries have interest in balancing China's rise.
Japan and India could work together but given their limits on projecting power and locations on opposite sides of China, neither could help the other much if one found itself at war with China. China could use interior lines to hold off one power by savaging the other which would be largely incapable of helping the other.
Japan has little ground power and no ability to project naval and air power to the Indian Ocean to directly help India fight China; India has a lot of ground power but no ability to project naval and air power to the South China Sea or East China Sea to directly help Japan.
That's where America comes in. America has the power and reach to knit together a lot of separate centers of power into a more coherent front to resist Chinese efforts to use their power to dominate their region at the expense of neighbors who would prefer not to be dominated by China.
This ability to join potentially isolated centers of resistance to Chinese ambitions makes the various local centers less likely to give in to Chinese pressure and instead hold their position in the line.
Obviously, American power is the key. Otherwise what practical help would Japanese-Indian military exercises provide?