WITH A $1 billion oil rig the size of a football field, China has literally laid down a new marker in its ambition to dominate the South China Sea — and challenged President Obama’s “rebalancing” policy in Asia, only weeks after the president’s tour of the region. The rig is about 130 miles off the coast of Vietnam, in waters that Vietnam claims as an exclusive economic zone under international law. China’s claim is more tenuous, but it is backed up with a flotilla of some 80 ships that for a week have engaged in a dangerous contest of ramming and water-hosing Vietnamese vessels.

The message of the deployment is as simple as it is provocative: The regime of Xi Jinping intends to unilaterally assert China’s sovereignty over almost all of the South China Sea without regard for the competing claims of five other countries or Mr. Obama’s newly restated commitment to uphold defense agreements with two of those nations. In that sense, the rig, like Russia’s invasion of Ukraine, is a fundamental challenge to the international order the United States has tried to preserve since the end of the Cold War.

China’s ambitions are described by an audacious map, dating from the pre-Communist era, that claims some 80 percent of the South China Sea and a number of island chains or waters also claimed by Japan, the Philippines, Taiwan, Brunei and Indonesia, in addition to Vietnam. For years Beijing has talked with those countries and others in Southeast Asia about establishing a code of conduct for the sea, and it discussed the possibility of joint development of oil and gas with Vietnam a few months ago.

The move of the oil rig appears to reflect a calculation that a more aggressive policy will not meet meaningful resistance from China’s neighbors or the United States. The target of the initiative, Vietnam, is particularly vulnerable, as it lacks strong military ties with Washington and is ruled by a Communist Party with a strong, pro-Chinese faction. The Vietnamese leadership has responded rather vigorously: In addition to the several dozen ships that are sparring with China’s near the rig, the regime allowed anti-Chinese demonstrations in Hanoi on Sunday, and its prime minister condemned China at a summit meeting of Southeast Asian nations.

China’s neighbors, however, predictably declined to take sides, issuing a communique that expressed “serious concerns” about “ongoing developments” without mentioning China. The Obama administration, for its part, issued a statement saying China’s action was “provocative” and “part of a broader pattern” to “advance its claims over disputed territory in a manner that undermines peace and stability in the region.” But the U.S. response is not likely to extend beyond rhetoric; a State Department official said in Hanoi last week that the administration took no position on the competing territorial claims.

Vietnam and the Philippines could bring a case against China at an international tribunal under the Law of the Sea treaty. But Beijing is likely to shrug off that form of pressure. Most likely it will continue to act unilaterally in the region until it meets concerted resistance, whether diplomatic or military. If the United States and its allies have a plan for that, it isn’t evident.