President Joe Biden announced Wednesday the planned withdrawal of all United States troops from Afghanistan, ending the longest war in U.S. history.

"It is time to end America’s longest war," Biden said from the rarely-used Treaty Room in the White House, the same place that former president George W. Bush announced the first airstrikes on Afghanistan in 2001. "It is time for American troops to come home."

"I am now the fourth American president to preside over an American troop presence in Afghanistan. Two Republicans. Two Democrats. I will not pass this responsibility to a fifth," the president said.

Biden will withdraw all U.S. troops from the country by Sept. 11, 2021, the 20th anniversary of the terror attack that drew the country into the conflict.

The president's announcement came from a deeply personal place, drawing from both his son Beau's deployment to the Middle East and from his time as vice president, when he took a trip to Afghanistan and told President Barack Obama that the U.S. should draw down.

"What I saw on that trip reinforced my conviction, that only the Afghans have the right and responsibility to lead their country," Biden said. "Endless American military force could not create or sustain a durable Afghan government."

The president said he called President Bush on Tuesday before making the announcement.

"He and I have had many disagreements over politics throughout the years, but we are absolutely united in our respect and support for the valor, courage and integrity of the women and men of the United States Armed Forces," Biden said.

President Biden also preemptively argued against critics who might say that now is not the time to leave Afghanistan due to lingering security concerns.

"So when will it be the right moment to leave? One more year? Two more years? 10 more years?" he asked. "Not now? That's how we got here."

Biden promised a "clear timetable for departure," which he said will now begin on May 1 — the deadline negotiated by the Taliban and the Trump administration — and end by Sept. 11. The president had hinted for weeks that he would let the May 1 deadline lapse.

Former President Obama, who paused the planned withdrawal of soldiers from Afghanistan in 2015, praised President Biden for the new timeline.

"President Biden has made the right decision in completing the withdrawal of U.S. forces from Afghanistan," Obama said in a statement. "After nearly two decades of putting our troops in harm's way, it is time to recognize that we have accomplished all that we can militarily, and that it's time to bring our remaining troops home."

Under Obama, U.S. Navy SEALs killed Osama bin Laden, the man behind the 9/11 terror attacks, in 2011. On Wednesday, Biden highlighted that killing as one of the original objectives of the war that has now been accomplished.

"We went to Afghanistan because of a horrific attack that happened 20 years ago," Biden added. "That cannot explain why we should remain there in 2021."

Still, some defense experts and members of Congress remain cautious about the drawdown.

"I’m looking forward to the briefing for Members of Congress next week to get more details," Rep. Elissa Slotkin (D-MI), who served in both the CIA and the Department of Defense, said in a statement.

"The two critical things I’ll be looking to hear come directly from the original goals of this war: first, what will the security architecture look like to ensure that terrorists don’t reconstitute and prepare attacks against us or our allies?" she said. “Secondly, what is the diplomatic plan for maintaining some insight and leverage in Kabul?" 

After his address from the White House, Biden visted Section 60 of Arlington National Cemetery, where veterans from the wars in Afghanistan and Iraq are buried, to pay his respects.

Earlier in the day, the president read out loud the number of troops killed in Afghanistan from a card he said he keeps in his pocket: 2,488. He highlighted the fact that he's the first president in decades to have a child who served in a war zone.

"Throughout this process, my North Star has been remembering what it was like when my late son Beau was deployed to Iraq," Biden said.

According to an administration official, the only U.S. forces to remain in Afghanistan will be those deemed necessary to protect diplomats in the country. An exact number had yet to be decided.

The extended timeline from May 1 to Sept. 11 will allow a safe and orderly withdrawal of American troops in coordination with NATO allies, the administration official added.

Ashraf Ghani, the president of Afghanistan, confirmed that he spoke to President Biden on Wednesday and said that the nation "respects the U.S. decision and we will work with our U.S. partners to ensure a smooth transition."

"Afghanistan’s proud security and defense forces are fully capable of defending its people and country, which they have been doing all along, and for which the Afghan nation will forever remain grateful," he added.

The president said that while the U.S. will not be involved in Afghanistan militarily, America will still support the country through diplomacy and humanitarian efforts.

"We will continue to support the Government of Afghanistan," Biden said. "We will keep providing assistance to the Afghan National Defense and Security Forces. Along with our partners, we are training and equipping nearly 300,000 personnel. And they continue to fight valiantly on behalf of their country and defend the Afghan people, at great cost."

Biden’s decision risks retaliation by the Taliban on U.S. and Afghan forces, possibly escalating the 20-year war. And it will reignite political division over America’s involvement in what many have called the endless war.

An intelligence community report issued Tuesday about global challenges for the next year said prospects for a peace deal in Afghanistan are “low” and warned that “the Taliban is likely to make gains on the battlefield. If the coalition withdraws support, the report says, the Afghan government will struggle to control the Taliban.

In a February 2020 agreement with the administration of President Donald Trump, the Taliban agreed to halt attacks and hold peace talks with the Afghan government, in exchange for a U.S. commitment to a complete withdrawal by May 2021.

Over the past year, U.S. military commanders and defense officials have said that attacks on U.S. troops have largely paused, but they say the Taliban have increased attacks on the Afghans. Commanders have argued that the Taliban have failed to meet the conditions of the peace agreement by continuing attacks on the Afghans and failing to totally cut ties with al-Qaida and other extremist groups.

When Biden entered the White House in January, he was keenly aware of the looming deadline and had time to meet it if he had chosen to do so. He began a review of the February 2020 agreement shortly after taking office, and has been consulting at length with his defense and military advisers as well as allies.

In recent weeks, it became increasingly clear that he was leaning toward defying the deadline.

“It’s going to be hard to meet the May 1 deadline,” Biden said in late March. “Just in terms of tactical reasons, it’s hard to get those troops out.” Tellingly, he added, “And if we leave, we’re going to do so in a safe and orderly way.”

The Associated Press contributed to this report.