IF THERE is one lesson that all nations with successful ballistic missile programs have learned, from the dawn of the missile age until today, it is that practice makes perfect. Testing is essential, and no one succeeds without it. North Korea’s latest missile launch did not reach the intercontinental range that its leader Kim Jong Un had threatened on New Year’s Day, but the event did show that North Korea is steadily testing and harvesting the know-how. The United States needs a new and serious strategy to meet this challenge.

The latest launch was a solid-fuel missile , lofted in a high trajectory, that flew 310 miles before splashing down in the Sea of Japan, also known as the East Sea. That’s a sufficient range to worry both Japan and South Korea, vital U.S. allies. The missile may be an adapted version of one known as the KN-11 that North Korea test-fired from a submarine last year. North Korea is clearly making progress in the technology of solid-fuel missiles, which are quicker to launch and require less in the way of support facilities. The weekend test was carried out on land from a mobile launching vehicle with tanklike tracks; mobility could give the missile more capability to evade detection. As always with North Korea, there are important unknowns, such as whether the solid-fuel engines or designs are indigenous or somehow imported.

Prudently, President Trump refrained from a rash response of the kind he delivered in January, when he tweeted: “North Korea just stated that it is in the final stages of developing a nuclear weapon capable of reaching parts of the U.S. It won’t happen!” There is nothing to be gained right now by drawing red lines. In a brief appearance before reporters late Saturday, Mr. Trump wisely emphasized the United States’ support for Japan while standing alongside Prime Minister Shinzo Abe. The tone of their meeting, as well as the recent phone call to President Xi Jinping of China, suggest Mr. Trump is attempting to soothe worries in Asia over his isolationist campaign rhetoric.

Now, Mr. Trump needs to focus on creating a strategy to deal with North Korea. On Monday he acknowledged it is “a big, big problem” and vowed to deal with it “very strongly.” There is no plausible military solution that does not risk a major war on the peninsula. This points toward negotiation. Two recent studies — one by the Council on Foreign Relations, the other by the Asia Society and the University of California at San Diego — have suggested an approach of pressuring North Korea to freeze its nuclear and missile programs. The United States must be willing to use sticks — such as sanctions and deploying missile defenses in South Korea — as well as carrots to end the North Korean threat. These studies, and many others, have emphasized how China is central to any such strategy. Mr. Trump, with his oft-touted dealmaking skills, enjoys a fresh chance to put the matter high on the agenda with China and tackle one of the world’s most pressing security problems. He should waste no time in doing so.