China said it will sanction three American defense contractors over proposed arms sales to Taiwan, retaliating against U.S. efforts to deepen security ties with the island democracy that Beijing claims as its territory.

Chinese Foreign Ministry spokesman Zhao Lijian said Monday that Beijing has decided to impose sanctions on Lockheed Martin Corp., Boeing Co.’s defense division, and Raytheon Technologies Corp., as well as other U.S. entities involved in the planned $1.8 billion weapons package.

The U.S. State Department last week approved proposals to sell missiles, rocket artillery, aerial reconnaissance sensors, and related gear to Taiwan—the Trump administration’s latest effort to put pressure on China through closer defense ties with Taipei.

China “firmly opposes” and condemns U.S. arms sales to Taiwan, which “severely damage Chinese sovereignty and security interests,” Mr. Zhao said at a routine briefing on Monday, calling on Washington to cease weapons deals and military cooperation with Taipei.

The news of the week in context, with Tyler Blint-Welsh.

While portraying the sanctions as necessary measures to safeguard China’s national interests, Mr. Zhao didn’t elaborate on the specifics or timing. He had similarly withheld details in July when he announced sanctions on Lockheed Martin in response to Washington’s approval of a $620 million upgrade package for Taiwan’s Patriot surface-to-air missiles.

China’s Communist Party has long sought to assimilate Taiwan with the Chinese mainland, saying it will use force if necessary. Beijing has ramped up patrols and drills around Taiwan this year, in what Taiwanese authorities describe as intimidation tactics.

Taiwanese President Tsai Ing-wen has pledged to strengthen the island’s armed forces, including by acquiring asymmetric combat capabilities to counter Beijing’s raw advantages.

Lockheed Martin, the world’s largest defense company by sales, said its presence in China is limited, adding, “Foreign military sales are government-to-government transactions and we work closely with the U.S. government on any military sales to international customers.” Boeing Defense, Raytheon, and the U.S. State Department didn’t immediately respond to requests for comment.

The sanctions are expected to have limited impact, as U.S. defense companies are broadly barred from making military sales to China. Lockheed Martin has sold civilian helicopters to Chinese buyers through its Sikorsky Aircraft Corp. unit. China is a key market for Boeing commercial aircraft, but the planned sanctions target only the company’s defense arm.

China and the U.S. have been exchanging blows as ties deteriorate over issues spanning trade, technological competition, global influence, and Washington’s cozying up with Taiwan.

Chinese officials have described Taiwan as the most sensitive issue in U.S.-China relations, and have repeatedly warned Washington not to use Taipei as leverage against Beijing. The Communist Party has sought control of Taiwan since Mao Zedong’s forces seized power in mainland China in 1949 and drove Chiang Kai-shek to relocate his Nationalist government to the island.

Though Washington maintains formal diplomatic ties with Beijing rather than Taipei, the U.S. is Taiwan’s main arms supplier. The 1979 Taiwan Relations Act commits the U.S. to provide defensive weaponry and treat threats to Taiwan as matters of “grave concern.”

The latest proposed U.S. arms sales, announced by the Defense Security Cooperation Agency last Wednesday, include a roughly $1 billion package of air-to-ground cruise missiles from Boeing, $436.1 million in mobile rocket launchers from Lockheed Martin, and $367.2 million in aircraft reconnaissance pods from Raytheon’s Collins Aerospace unit.

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