BACK IN January, President Obama rationalized his refusal to act in Syria in part by asking, in an interview with the New Republic, “How do I weigh tens of thousands who’ve been killed in Syria versus the tens of thousands who are currently being killed in the Congo?” Since then, his administration has supported a vigorous campaign of diplomacy and military intervention to stop the bloodshed in . . . Congo. This worthy effort now faces its first serious test.

A United Nations-sponsored conference in February produced a peace framework; in March the U.N. Security Council authorized a 3,000-strong “intervention brigade,” the first in U.N. history, to carry out offensive operations against armed groups. The force, composed of troops from South Africa, Malawi and Tanzania, is due to be fully deployed by next month — and it appears that its services will soon be needed. This month fighting has erupted between the Congolese army and a rebel group called M23 after months of relative calm. Thousands of people were forced to flee their homes in North Kivu province, where there are already nearly 1 million displaced civilians.

The scale of recent bloodshed in Congo is in no way comparable to that in Syria, but it stems from a chronic conflict that has repeatedly convulsed Africa’s Great Lakes region. With the Congolese government in faraway Kinshasa unable to control the region, neighboring countries — beginning with Rwanda — have repeatedly intervened. Rwanda originally sought to protect itself from Hutu militias that fled its territory after carrying out a 1994 genocide, but over the years it has developed economic interests in Congo and close ties with Congolese Tutsis.

According to a new report by Human Rights Watch, Rwanda is backing M23 despite its commitment at the February peace conference to stop sponsoring Congolese militias. The report says M23 has carried out scores of murders and rapes since March. It is not the only offender: Government troops are also guilty of abuses, as are smaller militias allied with one of the two sides. M23 may be trying to gain advantage ahead of the U.N. force’s deployment, which is why it’s important that the force begin to act on its mandate as soon as possible.

The Obama administration continues to focus on the problem: Secretary of State John F. Kerry is due to lead a ministerial discussion on Congo at the United Nations on Thursday. Mr. Kerry can be expected to remonstrate in private with representatives of Rwanda — which unconvincingly denies links to M23 — but he ought to speak out publicly about the violations as well. The United States and European governments, longtime supporters of Rwanda, suspended some aid last year after M23 briefly seized the city of Goma. Now they need to threaten further sanctions, while also offering Rwanda incentives, including economic carrots, that will allow it to beat a face-saving retreat from Congo once and for all.