On Friday, O'Brien said that Trump's Christmas suggestion was more of a wish than an order. "I'm going to engage in the rigorous analysis of the situation based on the conditions and the plans that I am aware of, and my conversations with the president", Milley said.
He said Trump's call for troops to be home by Christmas was more sentimental, that all presidents have called for the same thing in past wars.
"If the conditions permit it, we'd love to get people out earlier, and I think that's the desire that the president was expressing", O'Brien said.
O'Brien reiterated his announcement made last week that the United States was "going down to 2,500 troops" in "the early part of next year", adding that the US would love to exit earlier "if the conditions permit it".
"Troop withdrawals should be based on local conditions, not domestic politics", said Richard Haass, a former senior State Department official under George W. Bush, in a tweet. On Thursday US envoy Zalmay Khalilzad said the insurgents had agreed to reduce attacks and casualties. The officials spoke on condition of anonymity to discuss internal deliberations. They also worry that a hasty withdrawal could force the U.S.to leave behind sensitive military equipment.
In webcast comments to the Aspen Institute, Robert O'Brien added that Trump's recent tweet about bringing all USA troops home from the 19-year-old conflict by Christmas reflected a "desire", not an actual plan.
"If the USA withdraws, then they can come back by force", he said of Taliban fighters.
"Robert O'Brien or anyone else can speculate as they see fit". I am not going to engage in speculation.
"That was the decision of the President on a conditions-based withdrawal", Milley added, saying, "The key here is that we're trying to end a war responsibly, deliberately, and to do it on terms that guarantee the safety of the United States vital national security interests that are at stake in Afghanistan".
The Pentagon has not yet responded to questions about O'Brien's comments.
"It's not my practice to speculate", O'Brien said.
When asked about the different positions by Milley, Trump, and himself, O'Brien didn't name Milley, but added, "I wasn't speculating then, I wasn't speculating today".
Clarifying the Trump administration's stance after conflicting accounts from the president himself, his advisors and the Pentagon, O'Brien said United States troops would be at around 4,500 in the coming weeks, and further reductions would continue after that.
The US government has offered no details as to what the "re-set" would entail, but Khalilzad said it would mean "reduced numbers of operations".
"I can guarantee you".
'That's the order of the commander-in-chief, that's not speculation, he noted.
Milley said any drawdown past 4,500 was "conditions-based" and depends on progress in peace talks. "They might see it in their advantage".
America's exit from Afghanistan after 19 years was laid out in a February agreement Washington reached with the Taliban.
O'Brien even held out the possibility that USA forces might work together on counterterrorism operations with the Taliban against Islamic State Khorasan (ISIS-K), the local branch of the Islamic State group, a remarkable statement from a senior official of a country whose forces have waged an nearly two-decade-long counterinsurgency and counterterrorism campaign against the Taliban.
USA officials also say that the US has critical classified equipment at bases in Afghanistan that needs to be moved out, which will take time.
Trump has sought a full withdrawal of troops from Afghanistan.