President Donald Trump says he will stay the course in Afghanistan, pleasing advocates of military intervention but angering key elements of his conservative, non-interventionist base.

Trump announced Monday evening that he will broaden the mandate of U.S. soldiers in Afghanistan to prevent a defeat in the war that has dragged on for 16 years. His aides have said he would approve an increase in U.S. troops from an estimated 8,400 today up to about 12,400. But Trump declined to provide numbers in his address, although he said there would be no "blank check" and promised, "In the end, we will win." He also didn't describe how he would define success in Afghanistan.

Conservatives are upset because he is violating a campaign promise to get out of Afghanistan. And Trump conceded this in his prime-time speech Monday evening. "My original instinct was to pull out, and historically I like following my instincts," Trump said. "But all my life, I've heard that decisions are much different when you sit behind the desk in the Oval Office."

In effect, he admitted breaking his word and changing his mind. He said he had concluded that "a hasty withdrawal would create a vacuum for terrorists, including ISIS and Al Qaeda." This was substantively the same overall conclusion reached by his predecessor, Barack Obama, whom Trump had severely criticized for taking the wrong approach in Afghanistan. Among the remaining areas of difference, however, is that Obama set specific deadlines for troop withdrawals while Trump prefers making decisions regarding troop strength based on conditions on the ground.

The influential Breitbart web site, an ultra conservative outlet now run by Steve Bannon, who was fired last Friday as White House chief strategist, blasted Trump. "Trump reverses course, will send more troops to Afghanistan. Defends flip-flop in somber speech," read a Breitbart headline. The lead article, by Breitbart's Pentagon correspondent Kristina Wong, said, "The speech was a disappointment to many who had supported his calls during the campaign to end expensive foreign intervention and nation-building."

Gov. John Kasich of Ohio, who ran unsuccessfully against Trump for the Republican presidential nomination last year, issued a statement that, "Sixteen years and the lives of over 2,000 American heroes are more than enough of a price to have paid to eradicate a terrorist sanctuary. America cannot afford to make an open-ended commitment of further lives and treasure to the improbable proposition of building a cohesive nation in Afghanistan."

Sen. Rand Paul, R-Ky., who also ran against Trump, told reporters, "The mission in Afghanistan has lost its purpose, and I think it is a terrible idea to send any more troops into that war."