After months of intense preparations and weeks of extensive deliberations, the United States decided to effectively challenge China's sovereignty claims in the Spratly chain of islands by deploying a guided-missile destroyer, the USS Lassen, within the 12-nautical-mile radius of Chinese-occupied Subi Reef.

The United States has tried to justify its decision by invoking international law, particularly the United Nations Convention on the Law of the Sea (Unclos) and the strategic necessity to preserve freedom of navigation across the contested waters.

The Obama administration's decision to deploy a destroyer rather than, say, less lethal patrol vessels was certainly meant to send a strong message that the US is committed to serving as an anchor of stability in Asia.

The decision has been warmly welcomed by US allies, who have anxiously opposed the prospects of Chinese domination in the South China Sea, a vital artery of global trade and energy transport.

Manila, which has requested US assistance to protect its supply lines in the area against Chinese harassment, was particularly pleased to see a more robust pushback against Beijing's maritime ambitions.

As expected, China lashed out at the US, vowing to "resolutely respond to any country's deliberate provocations".

True to its words, Beijing dispatched its own destroyers to escort the US vessel out of the immediate surroundings of the feature under its control.

Professing neutrality vis-à-vis claimant states' sovereignty claims in the area, the Obama administration has repeatedly reiterated that its actions are not targeted against any specific country.

It has promised to conduct these so-called "freedom of navigation" (FON) operations on a regular basis, also targeting features claimed by other claimant states such as Vietnam and the Philippines.

A closer look, however, suggests that China is the main target of the US' latest manoeuvre.

For the past two years, China has engaged in a frantic construction bonanza, reclaiming 1,170ha of land and artificially transforming low-tide elevations and rocks into full-fledged islands, some

of which are poised to host advanced airstrips and military facilities. China has built a 3km-long airstrip on the Fiery Cross, which is expected to serve as the command and control centre of Beijing's operations in the Spratlys.

Similar facilities are under construction in Subi and Mischief reefs. The US and its allies fear that China could leverage this sprawling network of facilities to impose its will and restrict freedom of navigation and overflight in the area.

By building lighthouses and other advanced facilities on previously submergible land features, China could also bolster and gradually gain international recognition for its sweeping territorial claims, well beyond its 200-nautical-mile Exclusive Economic Zone and continental shelf.

By deploying surveillance missions into the 12-nautical-mile radius of Chinese-held features, the Obama administration is effectively countering Beijing's sovereignty claims.

Land features such as the Subi and Mischief reefs are considered low-tide elevations, which are not entitled to their own maritime entitlement zones.

So quite ironically, the US, which has not ratified the Unclos, is using international maritime law to rein in China's (a signatory) ambitions in the area.

The US' FON operations, therefore, are also meant to prevent China from turning its sovereignty claims into fait accompli, as well as dissuade it from deploying advanced weaponries and military hardware to the features under its control.

More than ever, the US is now directly involved in the territorial disputes in the South China Sea.

Its latest challenge against China carries both the promise of reining in China's assertiveness or, alternatively, intensifying tensions in the region and undermining prospects for a diplomatic management of the South China Sea spats.

What is clear, however, is that allies such as the Philippines, who are bereft of their own independent deterrence against China, are pleased to see the Obama administration more directly involved in the disputes.

• The writer is a political science professor at De La Salle University in the Philippines.