American military commanders have argued during the monthslong policy assessment that the additional troops would enable the United States to reverse gains made by the Taliban and militant groups like the Islamic State’s Afghan affiliate, the Islamic State in Khorasan.
Administration aides, under orders to let Mr. Trump announce the details, hinted that any American commitment to increase force levels would require steps by the Afghans, like doing more to fight corruption.
Mr. Trump’s Monday evening speech will be his first nationally televised prime-time address since he spoke before Congress in January and follows a week of controversy over his reaction to the racially charged violence in Charlottesville, Va.
When it comes to Afghanistan, Mr. Trump entered office as the skeptic in chief, and any ramped-up engagement there poses political risks for the president, who rallied voters weary of war with his sharp criticisms of American involvement in the country.
“We should have a speedy withdrawal. Why should we keep wasting our money — rebuild the U.S.!” Mr. Trump tweeted about Afghanistan in January 2013, as he considered running for office in 2016.
The Afghanistan question has been the source of a long-running debate at the White House, Stephen K. Bannon, who was recently removed as a top Trump adviser, fought the military’s recommendation for more troops and backed a number of alternative options — including using private contractors instead of United States forces.
The decision on troops is just one component of a military and political plan for the region that Mr. Trump and his aides have been discussing for months, and it is politically important for the president to differentiate his approach from the Obama-era policies he sharply criticized.
Administration officials have been developing ways to try to pressure Pakistan to shut down the sanctuaries there for the Taliban, a goal Republican and Democrat…