Elizabeth Economy and Michael Levi are senior fellows at the Council on Foreign Relations and the authors of “By All Means Necessary: How China’s Resource Quest is Changing the World.”

The China National Overseas Oil Corporation (CNOOC) began drilling in Vietnamese-claimed waters last week, accompanied by more than 70 vessels, including armed Chinese warships. At first glance, this might look like merely another front in China’s quest for natural resources, which has taken Chinese companies to seemingly every corner of the earth.

Yet what is happening in the South China Sea is actually far more dangerous than what has come before — and the forces driving it go well beyond pursuit of energy riches. The United States needs to face up to the full magnitude of the Chinese challenge to have any hope of successfully confronting it. This means not only tough talk but also a willingness to take difficult action.

There has long been speculation that massive oil and gas deposits are locked beneath the South China Sea — 1.4 million square miles bordered by Brunei, China, Malaysia, the Philippines, Taiwan and Vietnam and claimed in part by all of them. According to the Chinese Ministry of Land and Resources, the area might contain as much as 400 billion barrels of oil, surpassing the bounties of the Middle East.

Most informed estimates, though, are much smaller. The U.S. Geological Survey estimated in 2010 that the region’s undiscovered oil (much of which will never be financially attractive to produce) totals a far smaller 11 billion barrels. It is difficult to believe that China would risk armed conflict for such modest stakes.

Two other forces are essential to understanding what is going on. One is nationalism: The drilling is taking place near the Paracel Islands, which sit within a disputed area of the South China Sea, roughly 120 miles from Vietnam’s coast and well within Vietnam’s 200-mile exclusive economic zone. But China claims the islands based on historical usage and effective exercise of sovereignty, having occupied them since 1974. Backing off from the Paracels would deal a blow to China’s prestige, while underlining Chinese control over the islands would strengthen the leadership’s legitimacy at home.

Chinese leaders are also motivated by a desire to control the sea lanes of the South China Sea. More than $5 trillion of trade passes through the increasingly crowded waters each year. That includes almost one-third of world seaborne oil trade and more than three-quarters of Chinese oil imports (as well as most of the oil destined for Japan, South Korea and Taiwan). The Chinese navy may be too weak to challenge U.S. dominance in key Middle East sea lanes, or even to exercise control over the critical Straits of Malacca, but by operating naval forces across the South China Sea it can gain greater confidence that the United States will not be able to disrupt its supplies.

Beyond these two motivations, it does not hurt that Chinese oil companies are eager to operate in the region. By cloaking its military excursion in commercial garb, Beijing might have hoped to defuse some of the inevitable opposition.

If so, that gambit has not paid off. China’s latest move, which came as a surprise to Vietnam and other nations, undermines Beijing’s insistence that strong relations within the region are its top foreign policy priority. It also calls into question China’s commitment to its current working-group talks with Vietnam on joint resource development in the South China Sea.

The United States has said it won’t take a stand on the sovereignty dispute and has called on the two parties to resolve their differences peacefully. This is not enough: The United States ought to call China’s bluff and make clear the real stakes. The United States and the Association of Southeast Asian Nations (ASEAN) should present a unified front in refusing to recognize unilateral assertions of claims in disputed territories.

Even more important, the United States must be prepared to give life to its rhetorical position. Although it does not have a treaty obligation to defend Vietnam, its rebalancing to Asia is premised on its role as the primary guarantor of stability in the Pacific. Chinese actions challenge that.

Vietnam has reiterated its commitment to peacefully resolve the dispute. If China does not reciprocate, the United States should be prepared to offer support to Vietnam through an increased naval presence. This would give Washington the ability to assess Chinese capabilities and to help de-escalate the situation. Other options, such as restrictions on CNOOC’s activities in the United States, could also be considered. If the United States can’t back up its words with actions, its credibility in promising to uphold peace and stability in the region will be gutted.