Whenever he leaves Foggy Bottom, Rex Tillerson will be remembered as one of the most consequential secretaries of state in recent history — but for all the wrong reasons.

Mr. Tillerson, who has clashed with President Trump and may be fired soon, has presided over the hollowing out of the State Department. The exodus of senior and rising Foreign Service officers will debilitate American diplomacy for a generation. It’s hard to imagine doing more damage to an institution and less good for America’s standing in the world in such a short period of time. Meanwhile, Mr. Tillerson’s sound instincts on the substance of foreign policy were routinely undermined by Mr. Trump and his incontinent Twitter feed.

Mr. Tillerson’s obsession with downsizing our diplomacy has colored his time at the State Department. Instead of defending the department against the White House’s proposed 30 percent budget cut, he embraced it. Fortunately, Congress stopped — for now — the gutting of our diplomatic, democracy and development programs. Mr. Tillerson imposed a hiring freeze, canceled programs to diversify the department’s personnel, left empty dozens of the most senior positions requiring Senate confirmation and encouraged officers to leave by dangling $25,000 buyouts.

Equally damaging, Mr. Tillerson’s insular management style alienated or marginalized many of the department’s most experienced hands. He and the small team around him seemed to view foreign policy professionals as the enemy — a “deep state” opposed to Mr. Trump’s agenda. In this they were profoundly wrong. Over the past 25 years, I’ve worked closely with hundreds of Foreign Service officers and civil servants through Democratic and Republican administrations. To a person, they take pride in checking their personal beliefs at the department’s door and working for the success of whatever administration they serve. I could not tell you the political affiliation of any of the officers with whom I served.