Acting Secretary of Defense Christopher Miller announced on Tuesday that the U.S. would draw its troop levels in both Afghanistan and Iraq down to 2,500 by Jan. 15, 2021.

Why it matters: The U.S. currently has roughly 4,500 troops in Afghanistan, so this will be a significant reduction even as it falls short of President Trump's promise to end America's military presence there altogether.

Background: Miller's predecessor, Mark Esper, and other senior Pentagon officials had opposed further troop reductions unless conditions in Afghanistan improved. Trump fired Esper last week, reportedlyin part because of his desire to speed up the withdrawal from Afghanistan.

Driving the news: Miller took no questions from the press after making the brief announcement.

• He noted he had discussed the decision with Afghan President Ashraf Ghani and NATO Secretary General Jens Stoltenberg, both of whom are wary of an expedited U.S. exit.
• It will be difficult for NATO to maintain its troop presence in Afghanistan — currently around 12,000, per the Washington Post — if U.S. capabilities and infrastructure are no longer in place.
• Stoltenberg said in a statement on Tuesday that "the price for leaving too soon or in an uncoordinated way could be very high.”

The state of play: Trump signed a deal with the Taliban this February that called for a U.S. troop withdrawal. That retreat was contingent on peace talks between the Taliban and the Afghan government, and on a promise from the Taliban not to allow terror groups like al-Qaeda to use Afghanistan as a base of operations.

• Those intra-Afghan talks began in September but have made little headway. Violence in Afghanistan has continued and in some cases escalated.

What to watch: Biden has also promised to pull U.S. troops out of Afghanistan, though he has mentioned the idea of leaving a counterterrorism force behind.

• Congressional Republicans, including Mitch McConnell, have cautioned Trump not to pull out entirely, arguing that would present a vacuum for terrorist groups to fill.
• Flashback: Trump's national security team convinced him in 2017 to commit to the fight in Afghanistan and actually increase troop levels, rather than exiting entirely as he'd vowed to do.

Worth noting: There are currently about 3,000 U.S. troops in Iraq, with that number already having been reduced from 5,200 in September.