PHILIPPINE Foreign Secretary Albert del Rosario was in Washington last week for a rather specific purpose: to seek U.S. support in his country’s growing territorial dispute with China in the South China Sea. Mr. del Rosario told us he was seeking a “clarification” of the mutual defense treaty between the Philippines and the United States; he would like a U.S. statement suggesting it applies to a gas-rich seabed the Philippines and China are contesting. His government also would like help in beefing up its navy, perhaps through the lease of patrol boats.

These are tricky requests for the Obama administration, which has been trying to avoid taking sides in the increasingly dangerous clashes between China and its neighbors over a huge and vital Asian waterway that Beijing — in apparent contravention of international law — claims entirely for itself. China would like the United States to stay out of its disputes with the Philippines, Vietnam, Malaysia and Brunei, so that it can deal with each of those weaker countries in turn. “I believe the individual countries are actually playing with fire,” Vice Foreign Minister Cui Tiankai said Wednesday, “and I hope the fire will not be drawn to the United States.”

Such menacing language makes clear why the United States needs to exert its influence. Up to one-third of global trade passes through the South China Sea, so preserving freedom of navigation is a “national interest,” as Secretary of State Hillary Rodham Clinton put it last year. As important is checking China’s impulse to bully its neighbors, including not only friendly but weak democracies such as the Philippines but also Japan, which has its own maritime disputes with Beijing.

The Obama administration has made gestures in this direction. In addition to Ms. Clinton’s statement — which she repeated last week — Defense Secretary Robert M. Gates pledged recently that “five years from now the United States’ influence” in Asia will be “as strong if not stronger than it is today.” After meeting with Mr. del Rosario, Ms. Clinton said the United States was committed to the defense of the Philippines and to providing it with weapons, though she would not comment on the U.S. response to a potential attack by China in the South China Sea.

Such rhetoric ought to be coupled with initiatives. Ms. Clinton has suggested the United States could play a role in fostering multilateral discussions on the South China Sea; Washington should press China to formalize a “code of conduct” with Southeast Asian nations for handling territorial disputes. Notwithstanding its neutrality on territorial disputes, the Obama administration could point out the ways in which China’s claims objectively are at odds with United Nations conventions. And if Mr. del Rosario’s government wishes to shift its long-standing defense cooperation with the United States from counterterrorism to the patrol and defense of its territorial waters, the Pentagon should be ready to cooperate.