Defense Secretary Ashton B. Carter delivers a speech at the International Institute for Strategic Studies Shangri-La Dialogue on June 4 in Singapore. (Wong Maye-E/Associated Press)

This month, on their way from Taiwan to Japan, a group of U.S. senators inadvertently flew over a set of islands that both the Chinese and Japanese claim as their own. The Chinese government was incensed.

I obtained a formal protest letter from the Chinese Embassy in Washington sent to the Senate Armed Services Committee this week demanding that no U.S. official ever again fly in a straight line from Taipei to Tokyo, which takes you over what the Japanese call the Senkaku Islands and the Chinese call the Diaoyu Islands.

“Diaoyu Dao has been an inherent territory of China since ancient times,” the Chinese Embassy wrote. “The American aircraft entered China’s territorial airspace in violation of international law and the norm of international relations, which was a serious provocation against China’s sovereignty and security. . . . China will staunchly defend its territorial sovereignty over Diaoyu Dao.”

The incident was just the latest signal that the Obama administration’s strategy to deter Chinese aggression and encourage China’s good behavior in the South and East China seas is falling short. President Obama has tried to build a policy on diplomacy, quiet warnings and restrained military gestures. It’s not working. China has rebuffed them all, and if it loses and then ignores an arbitration case shortly as expected, then it’s not clear his administration has any new tools in its toolbox.

China has laid claim to a number of islands in the South China Sea, building airbases on tiny spits of land while installing radar and missile batteries. Here's why. (Jason Aldag,Julie Vitkovskaya/The Washington Post / Satellite photos courtesy of CSIS)

Over the past few weeks, the administration has tried to maintain the image that its South China Sea strategy is viable. At the Shangri-La Dialogue in Singapore this month, top defense and military officials insisted that the policy still might bear fruit and restrained themselves from saying anything that might upset their Chinese counterparts.

Unlike last year, Defense Secretary Ashton B. Carter’s keynote speech at the conference contained no messages crafted to send tough signals to the Chinese officials in the room. He briefly criticized Chinese aggression toward the end of his remarks.

“China’s actions in the South China Sea are isolating it, at a time when the entire region is coming together and networking. Unfortunately, if these actions continue, China could end up erecting a Great Wall of self-isolation,” he said.

Asked directly how the United States plans to change China’s strategic calculation that maritime expansion is in its best interest, Carter had no real answer. He pointed to the upcoming decision by an international tribunal on a case between China and the Philippines as a “big opportunity” for the region to return to a rules-based international order and quickly added, “I wouldn’t single out China.”

The White House had made it clear to Carter’s staff that it wanted no disruptions ahead of the U.S.-China Strategic and Economic Dialogue, which was to begin only days later, several U.S. officials told me. Only one week earlier, the White House scolded Carter after he gave a speech more critical of China at the U.S. Naval Academy. The White House was not upset at the content of the speech, officials said, but resented that Carter made headlines and upstaged Obama’s visit to Hiroshima.

Even Adm. Harry B. Harris Jr., head of the U.S. Pacific Command, who last year coined the phrase “great wall of sand” to protest Chinese island reclamation, avoided any criticism of China at the Singapore conference. “We’ve seen positive behavior the last several months with China,” he told reporters, praising what he said were Chinese efforts to avoid incidents in contested waters.

Carter and Harris’s charm offensive was not returned in kind by the Chinese military officials at Shangri-La. Adm. Sun Jianguo, deputy chief of the joint staff development for China’s Central Military Commission, gave a fiery speech in which he declared that China had done nothing wrong in the South China Sea and has no intention of abiding by the ruling of the international tribunal Carter touted.

Sun then accused the United States of “openly flaunting its military force, and on the other hand pulling in help from cliques, supporting their allies in antagonizing China.”

As of yet, U.S. moves to respond to China’s expansion and militarization in the South China Sea have not persuaded China to change course. The United States has sailed ships through contested waters, deployed U.S. troops and equipment in the Philippines, held huge military exercises with India and Japan, moved two aircraft carrier groups into the region and invested millions of dollars in new security partnerships.

China continues to harden its military facilities on artificial islands it built in the South China Sea, has begun sailing navy ships in the Senkakus and has ramped up its harassment of fishing boats from other countries in disputed waters. China reportedly is planning new construction on disputed islands in the Scarborough Shoal, and U.S. officials worry that China will soon announce a new Air Defense Identification Zone in the South China Sea.

The diplomatic effort to get all Association of Southeast Asian Nations countries together to stand up to China had recent setbacks as well. Last week, the United States worked behind the scenes to get all of the ASEAN foreign ministers to issue a statement criticizing China’s actions in the South China Sea. They issued a very strong statement but then retracted it only hours later under heavy Chinese government pressure. Now the Chinese are working hard to cut a side deal with the Philippines by offering the new president in Manila a new railway system.

The Obama administration has never been willing to use the big tools at its disposal — for example, economic sanctions — to confront Chinese maritime aggression, and there’s no sign that reluctance will change.

“If there were economic consequences of some kind involved, we would be more likely to get their attention,” said Bonnie Glaser, a senior adviser for Asia at the Center for Strategic and International Studies. “But most people in this administration are reluctant to say we are going to put the whole relationship at stake over this. And when you get to the final months of an administration, you begin to lose leverage.”

To the unresolved problems the Obama administration will bequeath its successor — Syria, Iraq, Ukraine, North Korea — you can add China’s assertiveness in the South and East China seas.