Democrats argued Trump was proposing an open-ended commitment with no exit strategy or ceiling on US troops there.
"Tonight, the President said he knew what he was getting into and had a plan to go forward. Clearly, he did not," House minority leader Nancy Pelosi said in a statement. "The President's announcement is low on details but raises serious questions."
New York Democratic Sen. Kirsten Gillibrand said Trump's speech was "terribly lacking" in details, substance and "a vision of what success in Afghanistan looks like."
And Rep. Ruben Gallego, an Arizona Democrat and Marine Corps veteran, accused Trump of "repeating the mistakes of previous administrations."
"Tonight, the American people should have heard a detailed, realistic strategy with achievable objectives and measurable benchmarks," Gallego said. "Instead, we got only vague promises and wishful thinking."
In his speech Monday evening, Trump said the US would be victorious in Afghanistan while focusing on defeating the terrorists as opposed to nation-building. He declared his administration would not talk about troop levels in order to keep the enemy in the dark, a frequent refrain during his campaign.
Trump's proposal won praise from one of his biggest foreign policy critics in the Republican Party, Sen. John McCain.
McCain, who had threatened to give Trump his own Afghanistan strategy
if the President couldn't come up with one, called Trump's plan "a big step in the right direction."
"I believe the President is now moving us well beyond the prior administration's failed strategy of merely postponing defeat," McCain said in a statement. The Obama administration was frequently criticized by Republicans for putting timelines on troop withdrawals.
The Arizona Republican said the US now must keep up with sufficient resources in Afghanistan to succeed there, adding that Trump "must conduct himself as a wartime commander" and arguing the President should speak regularly to the American people about the war.
House Speaker Paul Ryan told CNN's Jake Tapper at a town hall forum immediately following Trump's speech
that he was "pleased with the decision."
Ryan said he believed he had heard a new doctrine Monday from Trump of "principled realism."
Sen. Bob Corker, the chairman of the Senate foreign relations committee, who was considered for a top administration post, also praised Trump's speech, saying in a statement that he supported the direction that was laid out, though he noted there were still unanswered questions about the capability of the Afghan government.
"While there are certainly substantial questions about whether Afghanistan has the capacity over time to provide stable governance to its people, this more focused plan provides the U.S. military with the flexibility it needs to help the Afghan military regain momentum," Corker said.