Updated at 9:45 p.m. ET
President Trump declared that a hasty withdrawal of U.S. troops from Afghanistan "would create a vacuum" and that America is "not nation-building again; we are killing terrorists."
In a nationally broadcast address from Fort Myer in Virginia, Trump said he shares the American people's "frustration" with the long-running war in Afghanistan and that his "original instinct was to pull out." But he said the security threats the U.S. faces are "immense."
Speaking before a largely military audience, Trump said, "From now on, victory will have a clear definition: attacking our enemies, obliterating ISIS, crushing al-Qaida, preventing the Taliban from taking over the country, and stopping mass terror attacks against Americans before they emerge."
Trump said the U.S. will "shift from a time-based approach to one of condition" and that he will not talk about numbers of troops or plans for future military activities.
Trump is expected to deploy about 4,000 more troops to Afghanistan and try to tighten expectations on its government and that of neighboring Pakistan, senior U.S. officials told NPR ahead of the speech.
The president's decision follows months of deliberation with top U.S. commanders, political advisers and even enlisted veterans of the nearly 16-year war.
As expected, Trump did not include any end date to the stepped-up American presence, the way President Barack Obama did when he announced a surge in troops there.
When asked before the speech how long the U.S. presence could remain in Afghanistan, a senior U.S. official responded, rhetorically, "How long have we been in Korea?"
U.S. troops have been posted south of the Demilitarized Zone since the end of hostilities there in 1953. U.S. forces have been fighting in Afghanistan since the 2001 invasion that followed the Sept. 11 terrorist attacks.
Trump is said to have dreaded arriving at the decision to deploy more U.S. troops. The Pentagon recommended new deployments earlier this summer, but Trump has delayed making that official.
Although he rarely mentioned Afghanistan as a presidential candidate, Trump was critical of the war effort before running for president and said the United States should cut its losses.
Trump has remained deeply skeptical about new deployments since his inauguration.
The top U.S. commander in Afghanistan, Army Gen. John Nicholson, warned Congress early this year that the war is in "stalemate," and he and Defense Secretary James Mattis have both acknowledged that Russia has begun supporting Taliban insurgent forces in the north.
But Trump's political advisers are said to have argued that increasing the American commitment to Afghanistan after nearly 16 years — and a peak of more than 100,000 troops under Obama — would amount to throwing good money after bad.
Mattis, national security adviser H.R. McMaster and White House chief of staff John Kelly, whose son was a Marine officer killed in Afghanistan, are understood to have strenuously made the opposite case. The Pentagon and the U.S. intelligence community have long warned that withdrawing American support for the Afghan government would hasten its collapse.
The Afghan government cannot afford the security apparatus the U.S. and other international donors have helped build since 2001. Also the Taliban and other insurgent forces have long been able to seek refuge outside Afghanistan's borders in Pakistan, where the leadership of the Taliban continues to make its headquarters.
The Taliban are pressing gains against Afghanistan's forces, especially in the south and east, and Nicholson says more troops are necessary to backstop them. More U.S. troops also will make it possible to train more local troops to replace those lost in combat; the Afghan rate of attrition is heavy.
NPR correspondent Tom Bowman contributed to this report.