In 2010, Barack Obama's vice president, Joe Biden, vowed that the United States would be "totally out" of Afghanistan "come hell or high water, by 2014." In 2014 Obama said that he would leave about 8,000 U.S. troops there after all, and made an agreement with the new Afghan president, Ashraf Ghani, that extended their stay "until the end of 2024 and beyond."
Donald Trump wasn't having any of that. Back in 2013 he tweeted "Our troops are being killed by the Afghanis we train and we waste billions there. Nonsense! Rebuild the USA." But it looks like the generals have now got to him with what passes for military wisdom.
On Monday Trump announced that he would be sending more U.S. troops to Afghanistan — probably around 4,000 — and that they would stay as long as necessary. He has a clever new strategy, too: "We are not nation-building again. We are killing terrorists." (I bet George W. Bush and Obama wish they had thought of that.)