The United States has good reason to push back more forcefully against China’s grab for power in the South China Sea, as Defense Secretary Ashton Carter did on a trip to Asia this week. Beijing has repeatedly ignored earlier warnings to moderate the aggressive behavior that is unsettling its regional neighbors and further undermining its relations with the United States.  On Friday, American officials disclosed that China had installed two mobile artillery vehicles on an artificial island it is building in the sea, which is rich in natural resources like oil and gas and where China clearly hopes to establish some form of hegemony.  The weapons are not considered a threat to American naval forces. Still, they reinforce fears that China intends to militarize the Spratly Islands, a collection of reefs and rocks also claimed by the Philippines, Vietnam, Malaysia and Taiwan, and use them to control the waterway’s shipping lanes and dominate its smaller neighbors.  China’s ambitions have become increasingly clear since 2012 when it publicly asserted a claim to 80 percent of the South China Sea. In recent months, photographic evidence from commercial satellites and American spy planes has left little doubt that China is moving with alarming speed to turn the Spratlys into more substantial land masses, complete with runways and harbors.  Some American officials now believe China regards its claims in the South China Sea as nonnegotiable. If so, that’s terrible news for the region but also ultimately for China, which claims it prizes stability but will find it impossible to realize its economic goals if Asia is in constant tension. China’s bullying on the South China Sea has already caused many Asian countries to forge closer defense ties with the United States.  Now, the Obama administration has decided to more firmly underscore America’s intention to remain a Pacific power and to ensure that the region and its waters remain accessible to all nations. That is a role the United States has played constructively for decades, promoting a stability that has allowed Japan, South Korea and other countries, including China, to develop. “There should be no mistake: the United States will fly, sail and operate wherever international law allows, as forces do around the world,” Mr. Carter said in his speech. He also called for “an immediate and lasting halt to land reclamation by all claimants.”  Although the administration would obviously prefer a peaceful resolution of all South China Sea disputes, it cannot allow China’s claims to go unchallenged. It sent a surveillance plane close to one of China’s artificial islands, is considering air and sea patrols that could go closer to disputed reefs and shoals, and is expanding military exercises with regional partners.  President Obama and President Xi Jinping of China plan to meet later this year. In the meantime, American officials and their Chinese counterparts must avoid any miscalculation that could lead to a direct confrontation.  A version of this editorial appears in print on May 30, 2015, on page A18 of the New York edition with the headline: Pushback in the South China Sea. Order Reprints| Today's Paper|Subscribe
The United States is asserting its right to operate militarily around the Pacific, including areas where China is trying to dominate.