WASHINGTON — President Biden has signed an order authorizing the military to once again deploy hundreds of Special Operations forces inside Somalia — largely reversing the decision by President Donald J. Trump to withdraw nearly all 700 ground troops who had been stationed there, according to four officials familiar with the matter.

In addition, Mr. Biden has approved a Pentagon request for standing authority to target about a dozen suspected leaders of Al Shabab, the Somali terrorist group that is affiliated with Al Qaeda, three of the officials said. Since Mr. Biden took office, airstrikes have largely been limited to those meant to defend partner forces facing an immediate threat.

Together, the decisions by Mr. Biden, described by the officials on the condition of anonymity, will revive an open-ended American counterterrorism operation that has amounted to a slow-burn war through three administrations. The move stands in contrast to his decision last year to pull American forces from Afghanistan, saying that “it is time to end the forever war.”

Mr. Biden signed off on the proposal by Defense Secretary Lloyd J. Austin III in early May, officials said. In a statement, Adrienne Watson, the National Security Council spokeswoman, acknowledged the move, saying it would enable “a more effective fight against Al Shabab.”

“The decision to reintroduce a persistent presence was made to maximize the safety and effectiveness of our forces and enable them to provide more efficient support to our partners,” she said.

Ms. Watson did not indicate the number of troops the military would deploy. But two people familiar with the matter said the figure would be capped at around 450. That will replace a system in which the U.S. troops training and advising Somali and African Union forces have made short stays since Mr. Trump issued what Ms. Watson described as a “precipitous decision to withdraw.”

The Biden administration’s strategy in Somalia is to try to reduce the threat from Al Shabab by suppressing its ability to plot and carry out complicated operations, a senior administration official said. Those include a deadly attack on an American air base at Manda Bay, Kenya, in January 2020.

In particular, the official said, targeting a small leadership cadre — especially people who are suspected of playing roles in developing plots outside Somalia’s borders or having special skills — is aimed at curtailing “the threat to a level that is tolerable.”

Asked to square the return to heavier engagement in Somalia with the American withdrawal from Afghanistan last year, following through on a deal Mr. Trump had made with the Taliban, the senior administration official argued that the two countries presented significantly different complexities.

For one, the official said, the Taliban have not expressed an intention of attacking the United States, and other militant groups in Afghanistan do not control significant enclaves of territory from which to operate and plan.

Given that Al Shabab appears to pose a more significant threat, the administration concluded that more direct engagement in Somalia made sense, the official said. The strategy would focus on disrupting a few Shabab leaders who are deemed a direct peril to “us, and our interests and our allies,” and maintaining “very carefully cabined presence on the ground to be able to work with our partners.”

Intelligence officials estimate that Al Shabab has about 5,000 to 10,000 members; the group, which formally pledged allegiance to Al Qaeda in 2012, has sought to impose its extremist version of Islam on the chaotic Horn of Africa country.

While Al Shabab mostly fights inside Somalia and only occasionally attacks neighboring countries, some members are said to harbor ambitions to strike the United States. In December 2020, prosecutors in Manhattan charged an accused Shabab operative from Kenya with plotting a Sept. 11-style attack on an American city. He had been arrested in the Philippines as he trained to fly planes.

Mr. Biden’s decision followed months of interagency deliberations led by the White House’s top counterterrorism adviser, Elizabeth Sherwood-Randall, over whether to accept the Pentagon plan, maintain the status quo or further reduce engagement in Somalia.

In evaluating those options, Ms. Sherwood-Randall and other top security officials visited Somalia and nearby Kenya and Djibouti, both of which host American forces, in October.

The administration’s deliberations about whether and how to more robustly go back into Somalia have been complicated by political chaos there, as factions in its fledgling government fought each other and elections were delayed. But Somalia recently elected a new parliament, and over the weekend, leaders selected a new president, deciding to return to power Hassan Sheikh Mohamud, who led the country from 2012 to 2017.

For months, American commanders have warned that the short-term training missions that U.S. Special Operations forces have conducted in Somalia since Mr. Trump withdrew most American troops in January 2021 have not worked well. The morale and capacity of the partner units have been eroding, they say.

Of each eight-week cycle, the senior administration official said, American trainers spend about three unengaged with partner forces because the Americans were either not in Somalia or focused on transit — and the travel in and out was the most dangerous part. Other officials have also characterized the system of rotating in and out, rather than being persistently deployed there, as expensive and inefficient.

“Our periodic engagement — also referred to as commuting to work — has caused new challenges and risks for our troops,” Gen. Stephen J. Townsend, the head of the Pentagon’s Africa Command, told the Senate Armed Services Committee in March. “My assessment is that it is not effective.”

Intelligence officials have raised growing alarm about Al Shabab over the past several years as it has expanded its territory in Somalia. In its final year in office, the Obama administration had deemed Al Shabab to be part of the armed conflict the United States authorized against the perpetrators of the Sept. 11 attacks.

Once Mr. Trump became president, he loosened controls on airstrikes there, and the Pentagon significantly escalated American combat activity. But shortly before leaving office, Mr. Trump ordered most American troops to pull out of Somalia — except for a small force that has guarded American diplomats at a bunker by the airport in Mogadishu.

On its first day in office, the Biden administration suspended a permissive set of targeting rules put in place by the Trump administration, instead requiring requests for strikes — except in self-defense — to be routed through the White House. (Africa Command also invoked that exception for strikes undertaken in the “collective” self-defense of Somali partner forces.)

That pause was supposed to take only a few months while the Biden administration reviewed how targeting rules had worked under both the Trump and Obama administrations and devised its own. But even though it has largely completed a proposed replacement described as a hybrid between the two preceding versions, final approval of that has stalled amid competing national security policy matters.

The military, for its part, has tried to continue training, advising and assisting Somali and African Union forces without a persistent presence on the ground, but gradually increased the length of shorter stays. During a visit to Somalia in February, General Townsend warned of the threat Al Shabab posed to the region.

“Al Shabab remains Al Qaeda’s largest, wealthiest and most deadly affiliate, responsible for the deaths of thousands of innocents, including Americans,” he said. “Disrupting Al Shabab’s malign intent requires leadership from Somalis and continued support from Djibouti, Kenya, the U.S. and other members of the international community.”