The facts:

• The duration: The war in Afghanistan kicked off in October 2001. By 2011, there were just under 100,000 U.S. troops on the ground. Now, just 8,400 troops remain in an advisory and counterterrorism role, continuing the longest war in U.S. history.
• The casualties: 2,393 American troops have died, though just 37 of those deaths have come over the two-and-a-half years since withdrawal. Additionally, 24,841 civilians have died since 2009 — when the UN began systematically documenting civilian casualties in Afghanistan.
• The cost: Tough to pin down. A recent study calculated the combined costs of Afghanistan and Iraq at just under $5 trillion. Another report estimated that the annual cost per troop in Afghanistan averaged $1.186 million.
• The state of play: The American commander in Iraq told Congress in February that the remaining Afghanistan force faced a "shortfall of a few thousand" troops in a continued "stalemate" against the Taliban, which is now allegedly assisted by Russia.

Why it matters:

If Trump sends more troops to Afghanistan, he risks prolonging one of the longest and costliest conflicts in U.S. history.