WASHINGTON — WHEN I began as the State Department’s envoy for closing the detention facility at Guantánamo Bay, many people advised me that progress was impossible. They were wrong.

In the two years before I started, on July 1, 2013, only four people were transferred from Guantánamo. Over the past 18 months, we moved 39 people out of there, and more transfers are coming. The population at Guantánamo — 127 — is at its lowest level since the facility opened in January 2002. We also worked with Congress to remove unnecessary obstacles to foreign transfers. We began an administrative process to review the status of detainees not yet approved for transfer or formally charged with crimes.

While there have been zigs and zags, we have made great progress. The path to closing Guantánamo during the Obama administration is clear, but it will take intense and sustained action to finish the job. The government must continue and accelerate the transfers of those approved for release. Administrative review of those not approved for transfer must be expedited. The absolute and irrational ban on transfers to the United States for any purpose, including detention and prosecution, must be changed as the population is reduced to a small core of detainees who cannot safely be transferred overseas. (Ten detainees, for example, face criminal charges before the military commissions that Congress set up in lieu of regular courts.)

The reasons for closing Guantánamo are more compelling than ever. As a high-ranking security official from one of our staunchest allies on counterterrorism (not from Europe) once told me, “The greatest single action the United States can take to fight terrorism is to close Guantánamo.” I have seen firsthand the way in which Guantánamo frays and damages vitally important security relationships with countries around the world. The eye-popping cost — around $3 million per detainee last year, compared with roughly $75,000 at a “supermax” prison in the United States — drains vital resources.