A ruling by the top court in the Philippines that opens the door to increased military cooperation with the United States could also bolster Manila's security ties with Tokyo as the Southeast Asian nation grapples with an increasingly bellicose Beijing in the South China Sea.

The Philippine-U.S. pact, which expands military exchanges and gives American troops access to Philippine bases, is "critical" to the U.S. rebalance to Asia, said Patrick Cronin, senior director of the Asia-Pacific Security Program at the Center for a New American Security (CNAS) in Washington. "At the same time, moving forward with the enhanced defense cooperation agreement will accelerate Japan-Philippine cooperation."

According to Cronin, the growing defense ties are likely to be incremental, but their pace may depend on what action Beijing takes.