Secretary of Defense James Mattis is now ending his “Mission Reassurance,” the first foreign trip by a Trump administration official. He spent two days in Seoul and is finishing up in Tokyo.

The SecDef has been issuing strong words confirming America’s commitment to defend South Korea and Japan. That’s important. Now, however, it’s time for President Donald Trump to back up the reassurances with stiff economic sanctions on China for destabilizing North Asia.

Mattis’s staff, sounding pitch-perfect, characterized his inaugural foreign tour as a “listening trip.”

Yet the former Marine Corps four-star general was also there to speak. “I want there to be no misunderstanding during the transition in Washington that we stand firmly, 100 percent shoulder-to-shoulder with you and the Japanese people,” he said on Friday to Japanese Prime Minister Shinzo Abe.

“We stand with our peace-loving Republic of Korea ally to maintain stability on the peninsula and in the region,” Mattis said earlier in the day in Seoul. “America’s commitments to defending our allies and to upholding our extended deterrence guarantees remain ironclad: Any attack on the United States, or our allies, will be defeated, and any use of nuclear weapons would be met with a response that would be effective and overwhelming.”

What is neither effective nor overwhelming is America’s response to provocative Chinese actions directed against Seoul. For more than a year, Beijing has been trying to prevent South Korea from basing on its soil the Terminal High Altitude Area Defense system, designed to shoot down incoming missiles. From a location in the South, THAAD, as the Lockheed Martin-built system is called, can protect South Korea, Japan, Guam, and the United States. Beijing objects to deployment as it is worried that THAAD’s powerful radars can peer over the border into China and hit its missiles as well as North Korea’s.

Beijing has pulled out the stops in its campaign against Seoul, threatening to cut diplomatic relations and issuing media tirades. Moreover, it has used the Chinese economy as a club. It has, for instance, barred South Korea’s K-pop groups from performing in China, ended charter flights to the South, limited Chinese tourists going there, and banned the import of South Korean cosmetics.

And Beijing has gone after Lotte Group. The South Korean chaebol, the country’s fifth largest, is thinking of swapping land, a golf course, with the country’s Ministry of Defense so that the government can get a suitable parcel for the first THAAD battery.

There is talk of a boycott of Lotte Chinese outlets and “continuous sanctions.” Last November, the Chinese government also ordered an audit of an affiliate of Lotte and a fire-safety inspection of a Lotte store.

So far, the pressure on the conglomerate is working. On Friday, the board of directors of Lotte International, the group unit that owns the land in question, again deferred a decision to approve the deal. Perhaps significantly, the company did not announce a future board meeting.

In response, Seoul has stopped approving visas for Beijing’s Confucius Institutes in South Korea. That’s a brave step, but the South does not have the heft to significantly affect Beijing’s calculus on the matter.

The United States, however, does. The American market is critical to China, and closing it—or threatening to do so—would make Beijing rethink its intimidation of Seoul.

China is itself vulnerable to U.S. pressure. In 2015, China ran a trade surplus in goods and services of $336.2 billion. The surplus looks like it was slightly smaller last year, but it was nonetheless substantial. China could not sustain a sudden cut off of trade with the U.S.

Of course, America would be hurt as well, but the damage would be far smaller. The U.S. is not dependent on trade with China and its economy is far larger than China’s, thereby better able to withstand shocks. Last year, China’s gross domestic product, as reported by the official National Bureau of Statistics, was $10.83 trillion. America’s, according to the first estimate of the Bureau of Economic Analysis, was $18.86 trillion. In reality, the disparity is almost certainly larger due to Beijing’s overreporting.

Trump, while campaigning for the presidency, talked about a 45 percent tariff on Chinese goods to counteract the effect of Beijing’s increasingly predatory trade practices. Many objected to the cost of such a levy, but no cost is too high to get the earliest warning of a North Korean—or, for that matter, Chinese—launch of a nuclear weapon.

It’s outrageous that China has armed North Korea’s Kim regime and is now threatening “a small country”—Beijing’s demeaning term for South Korea—for trying to protect itself, but it’s understandable why it’s trying to get away with that. It is not inexplicable, however, why Washington allows the Chinese to get away with such intimidation.

It’s good for Mattis to issue reassurances to jittery American allies, but it would be even better for his boss to show real American commitment by imposing costs on China.

The best form of reassurance, after all, is action.