Just don’t say “thanks” to Washington. That’s the message from Communist China, piling on the rhetoric after the ruling party of the island democracy  of Taiwan dared to thank the Americans for approving more than $11 billion in arms sales for defense against marauding Chinese forces.

The thanks for the arms, issued by President Lai Ching-te’s Democratic Progressive Party, sent Beijing into rhetorical overdrive. Reporting on the expression of appreciation, the Beijing propaganda paper Global Times warned of a “firm response” if Taiwan forces dare cross a “red line” in pursuit of “Taiwan independence.”

The outburst from Beijing reflects rising tension over Mr. Lai’s defense buildup even as President Trump looks forward to seeing China’s party boss, Xi Jinping, at Beijing in April. It was the Department of State, not the White House, that disclosed Washington’s approval of the enormous arms buildup, all in accordance with the Taiwan Relations Act passed in 1979 after President Carter transferred American recognition of Chinese mainland rule to Beijing from Taiwan.

The Republic of China was delighted — and obviously relieved — by the arms deal even as Mr. Trump goes back and forth with  Beijing over tariffs designed to cut down China’s enormous trade surplus with America. The weapons covered in the sale, ranging from anti-tank and anti-ship missiles to parts for helicopters, serve Taiwan’s goal of being able to defend the island like “a porcupine” whose sharp quills could deter a far stronger attacker.

The arms sale, the largest in a long list of American agreements over the past few decades  to guarantee Taiwan’s military strength, fortifies its defenses at a time when China is rapidly building up its own forces. Taiwan leaders fear Mr.  Xi is plotting to try to overrun the island in the next two years.

Although Washington had to break off formal diplomatic relations with Taiwan when Carter turned toward Beijing, it remains committed to defense of the Taiwan regime, with which it maintains ties through “institutes” at Taipei and Washington.  

An undersecretary of defense, Elbridge Colby, has been a driving force for Taiwan’s defense while Mr. Trump courts Mr. Xi in pursuit of an understanding on trade. It’s still not clear how or even whether Messrs. Trump and Xi will face the Taiwan issue when they meet four months from now.

China’s state news agency, Xinhua, quoted a Chinese foreign ministry spokesman, Guo Jiakun, denouncing the arms sale as “a dangerous act” that “grossly violates the one-China principle” and “infringes on China sovereignty, security and territorial integrity, undermines peace and stability in the Taiwan strait and sends a gravely wrong signal to ‘Taiwan independence’ separatist forces.”

Mr. Lai, a strong advocate of self-rule for Taiwan, has said he is not looking for nationhood for Taiwan. Still, he places the island within a chain of much larger states, from Japan to the Philippines, that see Beijing’s power as a challenge if not an immediate threat.

Taiwan’s National Security Bureau this week said America and its allies “are reinforcing defenses along the First Island Chain” and expanding joint exercises and patrols “to bolster collective deterrence,” according to the Taiwan News. “These deployments,” it said, “have significantly boosted the US and its allies’ ability to control and deny access to key maritime areas.”

Warning that “reunification of the motherland is unstoppable,” Beijing’s Taiwan office called on “all sectors of Taiwan” to recognize “the evil essence of bottomless ‘selling out Taiwan and provoking war,'” according to the Global Times.