South Koreans protest a visit of Gen. Mark A. Milley, the U.S. Army chief, at the Defense Ministry in Seoul on Friday. Milley arrived Wednesday to discuss the Terminal High Altitude Area Defense antimissile system. (Ahn Young-joon/AP)

As the United States has looked for various ways to punish North Korea for its nuclear intransigence, one factor has remained constant: China.

Financial transactions, mineral exports, luxury imports — all of them come through China, making the nation’s enforcement of international sanctions critical to their success.

A once-reluctant Beijing had shown more willingness to inflict pain on Pyongyang this year. But that could be changing, the unintended result of the United States’ and South Korea’s decision to move an antimissile battery onto the peninsula to protect against North Korea.

“China has been complaining about this at the highest levels,” said Scott Snyder, a Korea expert at the Council on Foreign Relations. “Will this be a factor that diminishes their implementation of U.N. Security Council resolutions?”

President Park Geun-hye’s administration in Seoul had been equivocating on whether to host a Terminal High Altitude Area Defense (THAAD) antimissile unit for the U.S. military, fearful of angering Beijing. China is South Korea’s biggest trading partner, with its bilateral trade worth twice as much as that with the United States.

But North Korea’s recent actions — its nuclear test and long-range missile launch at the beginning of this year — prompted Seoul to agree in July to host the battery. Early Wednesday, the North was reported to have test-fired a submarine-launched ballistic missile off its east coast.

Beijing, which views the THAAD deployment as a way to keep China in check, has taken a range of actions recently to express its displeasure. The United States has a “strategic plot” to destabilize regional security, the state-run People’s Daily said in an editorial shortly after the decision.

So far this month, Beijing has suspended operations of the travel agency that helped South Koreans get multiple-entry visas for China — a visa used especially by those doing business, with 300,000 issued a year — and tourism has been hit.

One tour-bus company told South Korea’s CNBC channel that 30 to 40 percent of reservations had been canceled, while the Dong-A newspaper quoted travel firms as saying that Chinese tour package reservations for September and October were one-fifth lower than the same months last year.

Some South Korean actors who star in dramas that are popular in China have had their public appearances canceled, and some here even saw THAAD behind the Chinese city of Qingdao’s last-minute decision to cancel its participation in a chicken-and-beer festival in its South Korean sister city of ­Daegu.

Throughout North Korea’s decades of malfeasance, China’s overarching concern has been to ensure that its irascible neighbor remains intact and does not spill hungry refugees — or nuclear material — over its border.

But in the past few years, Beijing has shown signs of cracking down on the Kim Jong Un regime, most recently in March, when it supported the U.N. Security Council resolution to impose tough new sanctions — on mining exports in particular — on Pyongyang.

Hopes that China would follow through with implementation appear, however, to have been short-lived.

Chinese customs data showed that bilateral trade with North Korea was valued at $504 million in June, three months after the sanctions were imposed and the month before the THAAD announcement. That is almost 10 percent higher than the year before.

Trade has continued at a brisk pace in the past two months, said Lim Eul-chul of Kyungnam University’s Graduate School of North Korean Studies.

“Chinese companies that had held back on trading with the North have started to ship more goods after hearing the news that Seoul-Beijing relations have taken a turn for the worse over the THAAD issue,” Lim told the Yonhap News Agency.

The effort to squeeze the North has probably been neutralized by THAAD and the Chinese response, Snyder said.

The THAAD decision had coincided with other U.S. moves: to sanction Kim by name, to designate North Korea as a “primary money laundering concern” and to endorse an international ruling rejecting China’s claim to the South China Sea.

Even if each decision may have been correct on its own, Snyder said, “the four things together project an image of U.S. policy that is clearly not well received in Beijing.”

Kim Sook, a former South Korean ambassador to the United Nations, agreed.

“THAAD happened at the same time as the South China Sea decision, so China might have been doubly disappointed,” he said, adding that the South Korean decision to host the missile battery was the right one even if the timing was off.

“Strategically, our government has mishandled the case,” Kim said. “They hesitated for three years when they should have moved swiftly.”