The Philippines and Vietnam have been raising a storm about China's assertiveness in the South China Sea, but they're not the only ones voicing worry. Japan has been playing a relatively quiet, though significant role. It may not have a direct stake in the Paracel or Spratly Islands, but the world's third-largest economy has every interest in ensuring tensions don't swell. Increasingly Tokyo is acting on that interest.

At this week's Association of Southeast Asian Nations (Asean) Regional Forum in Phnom Penh, Japan's foreign minister intends to express serious concern at recent developments, press the parties to clarify their maritime claims and fast-track diplomatic solutions. While this intervention will be welcomed by Southeast Asian countries like Vietnam, it will almost certainly exacerbate friction in Sino-Japanese relations.

It's significant that Japan is willing to antagonize China over a territorial dispute in which Tokyo has no direct stake (notwithstanding its own, separate territorial frictions with Beijing). Tokyo has always kept an eye on the South China Sea, but it was not until tensions began to ramp up after 2008 that it felt the need to take a more proactive approach to the dispute. It's now going to the next level by directly confronting China.