'Twas the week before Christmas, when U.S. President Donald Trump issued another bombshell from the White House. With a single tweet, he decided to withdraw all U.S. troops from Syria over the coming months; the next day, his administration announced that the number of troops in Afghanistan, currently 14,500, would be halved.

According to The Times, Trump's decision to pull out of Syria "came during a phone call with President [Recep Tayyip] Erdogan of Turkey and took the U.S. defense leadership by surprise." After pointing out that the Islamic State had been 99 percent defeated, Erdogan reminded Trump of his own past statements listing IS as the only reason for the U.S. presence in Syria. As if on cue, Trump duly tweeted: "We have defeated ISIS in Syria, my only reason for being there."

The announced withdrawal from Syria and drawdown in Afghanistan met with consternation in Washington, among U.S. allies and within Trump's own Cabinet. Secretary of Defense James Mattis and Brett McGurk, the special presidential envoy for the global coalition to defeat IS, both resigned in protest. And yet, Trump's decision not only fulfilled a campaign promise; it also validates former U.S. President Barack Obama's own critiques of a "Washington playbook" that prescribes military responses to most foreign crises.