Chinese President Xi Jinping this month. (Leon Neal/Agence France-Presse via Getty Images)

PRESIDENT XI Jinping of China has been assertive since coming to power in 2013. Although China is governed in opaque ways, and he faces entrenched and competing interests, Mr. Xi has attempted to appear as a leader who is decisive and setting China’s direction. It is fair to assume that when Mr. Xi wants to get something done in the realm of the Chinese state and party, he can get results.

Mr. Xi promised in his Sept. 25 news conference with President Obama at the White House not to wage cyberwar on the United States for valuable commercial secrets and intellectual property. He declared : “Cybertheft of commercial secrets and hacking attacks against government networks are both illegal; such acts are criminal offenses and should be punished.”

Mr. Obama quite rightly asked whether words would be followed by actions. So far, it appears not.

Dmitri Alperovitch, co-founder of the cybersecurity firm CrowdStrike, which helps firms monitor assaults on their networks, wrote Oct. 19 that his firm has detected a continued effort by Chinese attackers to penetrate U.S. corporate networks — just the kind of behavior that Mr. Xi promised to stop. Mr. Alperovitch released a timeline of attempted intrusions into corporate networks — primarily tech and pharmaceutical firms — showing the attempts are unabated. Mr. Alperovitch added that the attackers appeared to be “affiliated with the Chinese government,” a conclusion enhanced by past experience with some of the intruders. Mr. Alperovitch also said that the attacks were thwarted — he is reporting on the attempts, not on actual thefts.

We were encouraged by Mr. Xi’s public pledge, on the grounds that it would provide a useful benchmark for China’s behavior. Nearly one month has passed since he made the promise. Mr. Xi ought to be able to change the behavior of China’s sprawling military and security establishment, which has been identified as the source of cyberattacks in recent years. It is possible he faces internal resistance to shutting down a program that has run expansively for a long time, or that some of the hackers are beyond the state’s control. But these cannot be excuses. Cyberattacks are often described as taking place at network speed. It is not as if they involve moving an expeditionary force across a battlefield. Mr. Xi ought to be able to say to his troops: Cut it out. And he must be judged on whether they do, or do not.

If Mr. Xi made the pledge and did not intend to keep it, then the United States must again be ready to impose sanctions. The theft of commercial secrets and intellectual property by China has gone on far too long. The cybersecurity agreement with Mr. Obama was a potential turning point in words that must be realized in deeds.