President Trump’s new strategy for the war in Afghanistan, which he laid out Monday night in a televised address, is intended to give a badly needed boost to the campaign to push back the Taliban, step up the fight against terrorism and reverse the steady deterioration of security that has allowed devastating bombings to shake Kabul.

The strategy, which would require several thousand more troops to implement, will likely help, current and former United States commanders said. It would allow American officers to more closely advise Afghan brigades, train more Afghan special operations forces and call in American firepower.

But even those who support Mr. Trump’s strategy cast his decision as the start of yet another challenging chapter that might, at best, enable Afghan forces to regain momentum on the battlefield over the next several years, not a quick fix for the problems that have bedeviled the region for nearly 16 years.

“I do not think many believe there could be an outright victory,” said Michael O’Hanlon of the Brookings Institution, who has been an advocate of sending more troops to Afghanistan. “But if President Trump can reverse the momentum, then he could arguably claim bragging rights and achieve at least a partial strategic success.”