The decision marked a turning point for USA involvement in the 17-year war, which Trump has consistently opposed, despite the best efforts of his generals to change his mind.
The Afghan military will dissolve without American support, Marine Lt. Gen. Kenneth F. McKenzie Jr., incoming Central Command chief, told lawmakers during a confirmation hearing earlier this month.
In 2002, some 130,000 global troops entered Afghanistan following the military intervention of U.S. forces to topple the Taliban government.
The Taleban insurgency has strengthened its grip over the past three years, with the government in Kabul controlling just 56 per cent of Afghanistan, down from 72 per cent in 2015, a U.S. government report showed. There has been more momentum now for talks than ever before, which Trump's decision significantly undermines.
"Frankly speaking we weren't expecting that immediate U.S. response", the official told AFP from an unknown location in northwest Pakistan. "We train them to the best of our abilities", said a Western diplomat from a Resolute Support member. "We are expecting more good news".
It is not clear if U.S. peace envoy Zalmay Khalilzad or the Afghan government had been aware of Mr Trump's plans.
Afghan President Ashraf Ghani recently announced that about 29,000 Afghan soldiers and police had been killed or wounded since 2015.
Presidential spokesperson Haroon Chakhansuri also said on social media that a United States withdrawal will not have a security impact.
On the other hand, she said, if they see it as a victory over the foreigners, the war will likely intensify, which could spur the U.S.to send in more troops again. "If we start giving concessions like this, it's not going to help".
"I believe the Taliban will see this as a reason to stall, and therefore it disincentivizes the Taliban to actually talk to the Afghan government, which it has refused to do", said Bill Roggio an Afghanistan analyst with the Washington-based Foundation for Defense of Democracies.
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"Afghanistan will go back to the Taliban era", she told AFP.
The decision apparently came after Mr Khalilzad met the Taliban in Abu Dhabi this week, part of a flurry of diplomatic efforts to bring the Taliban to the negotiating table with the Afghan government.
But a former senior State Department official familiar with the issue said that the Taliban representatives rejected a proposal by Khalilzad for a ceasefire and demanded that the talks focus on a USA withdrawal.
"I think the factors that are contributing to the resignation - maybe not all of it - but certainly the decisions in Syria and Afghanistan", Keane said. Trump reportedly told his advisers in 2017 that he wanted to withdraw troops from Afghanistan, but he ended up deploying 3,000 more at Mattis' direction, per Politico. Lindsey Graham, who has vocally led the bipartisan outrage over Trump's decision to withdraw all USA forces and diplomatic personnel from Syria. And it comes as Defense Secretary Jim Mattis announced plans to resign, citing differences with Trump over the value of America's alliances and leadership in the world.
Critics suggest the president's twin foreign policy decisions on Syria and Afghanistan could unspool a series of cascading and unpredictable events across the Middle East and in Afghanistan. The conditions in Afghanistan-at the present moment-make American troop withdrawals a high risk strategy.
In that year, more than 100,000 North Atlantic Treaty Organisation troops pulled out of the country and handed off security to Afghans. Since the withdrawal, what little forces remained in the country, retreated mostly to training and the advisory role.
Ghani said last month almost 30,000 Afghan security forces had been killed since the start of 2015, a figure much higher than anything previously acknowledged. Local ISIS cells have been behind some of the most deadly attacks on Kabul in recent years. It was unclear whether that figure referred to deaths or also included injuries. U.S. officials told the New York Times and Wall Street Journal that about half that number would be withdrawn.
The US nearly certainly would have to curtail its missions, something that could provide an opportunity for a resurgent Taliban to expand their offensives across Afghanistan. More than 2,400 American soldiers have also died in Afghanistan since the 2001 US -led invasion.
The Taliban insurgency has strengthened its grip over the past three years, with the government in Kabul controlling just 56 percent of Afghanistan, down from 72 percent in 2015, a United States government report showed.
Mr Trump privately has been grousing about U.S. military involvement in Afghanistan, telling an ally as recently as Wednesday, words to the effect of: "What are we doing there".
The Trump administration is planning to withdraw thousands of troops from Afghanistan, US media say.