WASHINGTON (NewsNation Now) — President Joe Biden on Thursday said the U.S. military mission in Afghanistan will conclude on Aug. 31, saying “speed is safety” as the United States seeks to end the nearly 20-year war.

“We did not go to Afghanistan to nation build,” Biden said in a speech to update his administration’s ongoing efforts to wind down the U.S. war in Afghanistan. “Afghan leaders have to come together and drive toward a future.”

Biden also amplified the justification of his decision to end U.S. military operations even as the Taliban make rapid advances in significant swaths of the country.

Biden had been under pressure from critics to give a more expansive explanation for his decision to withdraw.

The effort to further explain his thinking on Afghanistan comes as the administration in recent days has repeatedly sought to frame ending the conflict as a decision that Biden made after concluding it’s an “unwinnable war” and one that “does not have a military solution.”

“How many more, how many more thousands of American daughters and sons are you willing to risk?” Biden said to those calling for the U.S. to extend the military operation. He added, “I will not send another generation of Americans to war in Afghanistan, with no reasonable expectation of achieving a different outcome.”

Biden said he didn’t trust the Taliban but trusted the capacity of the Afghan military to defend the government.

Before his speech, White House press secretary Jen Psaki said Biden administration officials always anticipated an “uptick” in violence and greater turmoil as the U.S. withdrawal moved forward. She added that prolonging U.S. military involvement, considering former President Donald Trump had already agreed to withdraw U.S. troops from Afghanistan by May 2021, would have led to an escalation of attacks on American troops.

“The question fundamentally facing him was after 20 years was he going to commit more American troops to a civil war in Afghanistan,” Psaki said.

The United States last weekend abandoned Bagram air base, the longtime staging ground for U.S. military operations in the country, effectively ending America’s longest war. The Pentagon says the withdrawal of U.S. forces is 90% complete.

Washington agreed to withdraw in a deal negotiated last year under former President Donald Trump. Biden overruled military leaders who wanted to keep a larger presence to assist Afghan security forces and prevent Afghanistan from becoming a staging ground for extremist groups.

Instead, the United States plans to leave 650 troops in Afghanistan to provide security for the U.S. Embassy.

Biden’s order in April to pull out U.S. forces by Sept. 11 after 20 years of conflict has coincided with major gains by the Islamist militant Taliban movement against overwhelmed Afghan forces after peace talks sputtered.

Gen. Austin Miller, the commander of U.S. troops in Afghanistan, warned last week that the country may be headed toward civil war.

Biden’s administration is also grappling with its plan for expedited visas for Afghan people most at risk of being attacked by the Taliban, including translators who worked with foreign forces. Rights groups are pushing to add up to 2,000 vulnerable women to the list, and Biden is expected to mention women’s rights in his remarks.

Some Republicans are criticizing Biden for the pullout, although Trump had also sought to end American involvement in the war.

Biden met Afghan leaders at the White House on June 25 and said U.S. support for Afghanistan would continue despite the pullout.

“Afghans are going to have to decide their future, what they want,” he said at the time.

This comes as British Prime Minister Boris Johnson announced most British troops have been pulled out of Afghanistan ending its official role in the conflict.

British forces were first deployed to Afghanistan in 2001 following the September 11 attacks and played a major role in combat operations until 2014. A total of 457 British soldiers were killed in the country.

All British troops assigned to NATO’s mission in Afghanistan are now returning home,” Johnson said in a statement to parliament.

“For obvious reasons, I will not disclose the timetable of our departure, though I can tell the house (parliament) that most of our personnel have already left.”

Johnson outlined the change to Britain’s presence in Afghanistan, saying he did not underestimate the challenges facing the country and the government would continue to provide development assistance.

“I hope no one will leap to the false conclusion that the withdrawal of our forces somehow means the end of Britain’s commitment to Afghanistan, we are not about to turn away, nor are we under any illusions about the perils of today situation and what might lie ahead,” Johnson told parliament.

All reporting by Reuters. Reuters contributed to this report.