THE DEATH of four U.S. servicemen in Niger has prompted a flurry of questions inside and outside Congress about U.S. military operations in that country and in Africa more generally. Some seem misinformed: A couple of senators have protested that they were unaware of the extent of U.S. deployments in West Africa, despite regular Pentagon reports and briefings to Congress. Other questions appear to be based on unfounded speculation. While the incident that led to the soldiers' deaths is still under investigation, Pentagon officials say it does not result from, or augur, an expansion of U.S. combat missions in Niger or in Africa more generally.

About 800 of the 6,000 U.S. military personnel posted in Africa are now in Niger; most of the rest are at a base across the continent in Djibouti. But only in Somalia and Libya are U.S. forces authorized to carry out direct action against terrorist groups — and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said there would be no "mission creep." Instead, the Trump administration has announced its backing for a five-nation West African force to combat extremists and will continue training and intelligence missions.

The administration inherited this prudent approach from the Obama administration and has done little to alter it. But the debate sparked by the Niger incident nevertheless will be helpful if it motivates Congress to take up a long-neglected piece of business: updating the legal authorization for U.S. military action against terrorist groups. U.S. drone and commando strikes in Somalia, Libya, Yemen, Syria and other far-flung places are still conducted under the aegis of the 2001 Authorization for Use of Military Force (AUMF) passed by Congress after the 9/11 attacks.

As several senators pointed out during a hearing of the Foreign Relations Committee on Monday, few who voted for that law imagined it might be used to justify operations far from al-Qaeda's 2001 base in Afghanistan. Legal experts divide over the Obama and Trump administrations' judgment that the law covers action against the Islamic State, which did not exist in 2001. But senior Trump administration officials agreed during the hearing that a new AUMF would, at least, reinforce the political mandate for defeating Islamist terrorism.

The problem, as committee Chairman Bob Corker (R-Tenn.) observed, is bridging the gap between legislators who want to provide clear authorization for military action and those who want to constrain it. A bill sponsored by Sens. Tim Kaine (D-Va.) and Jeff Flake (R-Ariz.) would replace the 2001 AUMF while narrowing its authority; the Pentagon could not target new al-Qaeda or Islamic State groups unless they attacked U.S. forces, and Congress would have the option to block new operations with a vote of disapproval. In addition, the authority would expire after five years unless Congress reauthorized it.

The administration contends that the proposed process for designating new terrorist groups is too restrictive, though it's not clear it would have blocked the counterterrorism expansions that have occurred since 2001. The administration also objects to the sunset provision. But the absence of a time limit would allow the executive to wage war for decades across the world without congressional reauthorization. Whether or not that's constitutional, it's unwise.