Reversing his past calls for a speedy exit, President Donald Trump recommitted the United States to the 16-year-old war in Afghanistan, declaring U.S. troops must "fight to win." He pointedly declined to disclose how many more troops will be dispatched to wage America's longest war.

In a prime-time address to unveil his new Afghanistan strategy, Trump said Monday the U.S. would shift away from a "time-based" approach, instead linking its assistance to results and to cooperation from the beleaguered Afghan government, Pakistan and others. He insisted it would be a "regional" strategy that addressed the roles played by other South Asian nations - especially Pakistan's harboring of elements of the Taliban.

"America will work with the Afghan government as long as we see determination and progress," Trump said. "However, our commitment is not unlimited, and our support is not a blank check."

Although Trump insisted he would "not talk about numbers of troops" or telegraph military moves in advance, he hinted that he'd embraced the Pentagon's proposal to boost troop numbers by nearly 4,000, augmenting the roughly 8,400 Americans there now.

Before becoming a candidate, Trump had ardently argued for a quick withdrawal from Afghanistan, calling the war a massive waste of U.S. "blood and treasure" and declaring on Twitter, "Let's get out!" Seven months into his presidency, he said Monday night that though his "original instinct was to pull out," he'd since determined that approach could create a vacuum that terrorists including al-Qaida and the Islamic State would "instantly fill."

And while Trump has pledged to put "America First," keeping U.S. interests above any others, his national security advisers have warned that the Afghan forces are still far too weak to succeed without help. Even now, Afghan's government controls just half the country.

John McCain, the Senate Armed Services Committee chairman who'd criticized Trump for delays in presenting a plan, said the president was "now moving us well beyond the prior administration's failed strategy of merely postponing defeat." McCain also tweet praise from Trump's speech writing, "I commend @POTUS's new strategy for #Afghanistan. We must move past last admin's failed policy of postponing defeat." Senators Marco Rubio and Lindsey Graham, close allies to McCain on foreign policy, tweeted similar sentiments.

Breitbart News, now led by former Trump chief strategist Steve Bannon, who is critical of military interventionism blasted Trump in its lead headline immediately after the speech saying, "Trump reverses course, will send more troops to Afghanistan" and "Defends flip-flop in somber speech," punctuated with two words "Endless war."

House Democratic leader Nancy Pelosi said the speech was "low on details but raises serious questions."

"Tonight, the president said he knew what he was getting into and had a plan to go forward. Clearly, he did not," said Pelosi, D-Calif.

At its peak, the U.S. had roughly 100,000 in Afghanistan, under the Obama administration in 2010-2011. The residual forces have been focused on advising and training Afghan forces and on counterterror operations - missions that aren't expected to dramatically change under Trump's plan.

"I share the America people's frustration," Trump said. But he insisted, "In the end, we will win."