For the 11 years he served as chairman and C.E.O. of ExxonMobil, Rex Tillerson would often retreat to his office on the highest floor of the company’s sprawling headquarters in Irving, Texas. Dubbed the “God Pod” by employees, it was here that Tillerson ate private lunches with a close circle of subordinates, almost completely cut off from the rank and file. Nine months into his State Department tenure, Tillerson has re-created his former way of life with remarkable precision. His current redoubt is Mahogany Row, the seventh floor corridor of executive offices at the State Department, where he’s surrounded with a few familiar faces. Phone conversations with his former corporate equals—chief executives Randall Stephenson of AT&T, Doug McMillon of Wal-Mart, and Rich Templeton of Texas Instruments—dot his calendar. The secretary continues to eschew the traditional method of communication at the State Department, memoranda, in favor of PowerPoint presentations—a pervasive aspect of ExxonMobil’s corporate culture. Instead of dining with Trump at his Washington, D.C. hotel, or joining his boss on the links, Tillerson reportedly returns home to Texas or jaunts to Colorado to visit his parents.

Tillerson’s bubble is a management style, and a whole philosophy. His aggressive resistance to assimilation into the culture of the institution he now leads has been, he insists, by design. “The most important thing I can do is to enable this organization to be more effective, more efficient, and for all of you to take greater satisfaction in what you do day in and day out,” the secretary said in September. But now, after months of anecdotal rumblings about the hollowing out of the State Department, the secretary’s overhaul of the more than 225-year-old organization is quantifiable.

According to A.F.S.A., the labor union for the United States Foreign Service, the number of State Department employees that rank as either career ambassador or career minister—the equivalent of four- and three-star generals in the military, respectively—has fallen from 38 to 21 since January. Meanwhile, the number of minister counselors, the two-star commensurate, has dropped dramatically from 431 to 369 since right after Labor Day. Even future generations of foreign-service officers have taken a hit; the number of applicants registering to take the Foreign Service Officer Test this year will be less than half the 17,000 who registered just two years ago.

And the talent exodus is far from over. Tillerson, a veteran of massive reorganizations at Exxon and the Boy Scouts of America, is offering buyouts to the first 641 employees who agree to leave by April in an effort to get closer to his goal of reducing the department’s full-time American staff by eight percent, the State Department confirmed on Friday. But with a price tag of $25,000, the buyouts aren’t enough to float a new career, meaning it will likely be the most experienced individuals already contemplating retirement or top-flight talent that opt to leave. “The way this works is that often the best people have the most outside opportunities, and so what is happening is that the best diplomats are electing to leave,” Rob Berschinski, who served as deputy assistant secretary of state for democracy, human rights, and labor under Barack Obama, told me in a recent interview.

The department is losing more than just headcount. What’s walking out the door, say senior diplomats both still on the scene and departed, is a vast amount of knowledge that’s long been essential for managing America’s position in the world, of a kind that can’t be written in a book or stored in a database. Whether it can be replicated is a question that doesn’t have an answer yet.