The last US military planes have left Afghanistan marking the end of the United States’ longest war

(CNN) -- The last US military planes left Afghanistan, Commander of US Central Command, Gen. Frank McKenzie announced Monday at the Pentagon. The US departure marks the end of a fraught, chaotic and bloody exit from the United States' longest war.

The departure marks the first time in nearly two decades that the US and its allies have not had troops on the ground in Afghanistan and -- after $2 trillion in spending and nearly 2,000 US troops killed in action -- the pullout raises questions about the utility of a war that saw the service of parents and then their grown children.

A senior State Department official said Monday that the State Department will not have civilians on the ground once the US military leaves Afghanistan. "We are not going to have civilians on the ground once the military has left," they said.

The official said that they expected the US Embassy in Kabul to suspend embassy operations upon the end of the military retrograde, but said "that doesn't mean that we are suspending any commitments to American citizens in Afghanistan, to at risk Afghans, to the Afghan people."

Nearly 20 years after the US invaded Afghanistan to avenge the terror attacks of September 11, 2001, and strike at al Qaeda and the Taliban, which hosted Osama bin Laden, another American administration is leaving the country in the control of Taliban militants who still maintain close ties to al Qaeda and other terrorist organizations.

Even as Biden pulls the US from the country, Afghanistan looks likely to shadow him politically and engage him militarily -- on Monday, White House officials said the President is continuing the hunt for terrorists in the country, telling his military commanders to "stop at nothing" to avenge the deaths of 13 US service members at Kabul airport last week.

As of Monday, more than 122,000 people had been airlifted from Hamid Karzai International Airport since July, Pentagon spokesman John Kirby told reporters, including 5,400 Americans.

A senior State Department official said the department believes there are fewer than 250 American citizens currently in Afghanistan who may wish to leave, as US officials stressed a Taliban commitment to let Afghans leave the country after the US and allies' withdrawal. The State Department official put the number of American citizens who have left the country through evacuation flights or other means closer to 6,000.

"We believe there's still a small number who remain, and we're trying to determine exactly how many," the official told reporters Monday. "We are going through manifests of people who have departed, we are calling and texting and WhatsApping and emailing our lists, in an effort to have a more concrete figure regarding how many Americans may remain."

In the 24 hours leading up to Monday morning, 26 military C-17 aircraft lifted off from Kabul carrying 1,200 evacuees, according to Gen. Hank Taylor, the deputy director of regional operations for the Joint Staff, who spoke alongside Kirby at a Pentagon briefing earlier Monday. In total, 28 flights departed from Kabul airport in that 24-hour window, Taylor said.

In the same 24-hour period, the US conducted a drone strike that killed multiple civilians, including children, the Kabul airport was targeted by rocket fire, and military officials continued to warn of active, specific threats to the evacuation effort.

The "threat stream is still real. It's still active, and, in many cases, it's still specific," Kirby said at the Monday morning briefing when asked if another attack on the airport was still likely. Taylor added that military operations were continuing with a focus on the security of the US troops in Kabul, and the military would have capability to evacuate Afghans until the very end.

"We're taking it very seriously and we will right up until the end," Kirby said.

Along with the military exit, the US is pulling out all diplomatic representation, leaving open the question of whether it will formally recognize the Taliban as the rulers of Afghanistan.

The formal military and diplomatic "retrograde" is ending even as the US leaves behind Americans, some of whom did not want to leave and others who may have already left, according to State Department officials, as well as vulnerable Afghans who worked for the US military and now face possible Taliban retaliation.

That tragically unfinished business will become part of the broader political challenge that Biden faces as he enters the second half of his first year in office. As a devastating hurricane batters the South and the Covid-19 pandemic once again gains traction, Biden is also facing calls for an accounting of the Afghanistan withdrawal.

The airlift, which started as a seemingly haphazard and hastily organized effort, was scarred by the deaths of 13 service members last week and the death sentence hanging over Afghan translators who helped US troops and diplomats but were unable to escape the country. In addition, Biden's decision to leave will be shadowed by questions about whether and how well the threat of terrorism emanating from Afghanistan has really been addressed.

The President has already committed to prolonging some US engagement with Afghanistan, telling his military commanders that they should "stop at nothing" to make ISIS pay for the service members' deaths, Psaki said Monday.

I can tell you that the President has made clear to his commanders that they should stop at nothing to make ISIS pay for the deaths of those American service members at the Kabul airport," Psaki said at a White House press briefing.

Sunday's drone strike, targeting a suspected car bomb headed to the airport, killed nine members of one family, including six children, according to a relative of those killed who spoke to a local journalist working with CNN. US Central Command, which oversees US military missions throughout the region, had said earlier they were assessing the possibilities of civilian casualties.

As the US presence drew to a messy close, Blinken convened an international ministerial on Afghanistan, meeting virtually with members of the coalition present in Afghanistan and other allies and the United Nations Security Council met in New York.

The UN body was briefed on the latest news out of Afghanistan after France and Germany proposed a Security Council resolution "calling for safe passage for those leaving Afghanistan," that would include counterterrorism, human rights and humanitarian elements, diplomats told CNN over the weekend.

This story is breaking and will be updated.