Secretary of State Rex Tillerson might not have his job for much longer, but his tenure may well be regarded as the most consequential in postwar American history: not for what he built but for what he destroyed.

In only ten months, Tillerson, the former C.E.O. of ExxonMobil, has presided over the near-dismantling of America’s diplomatic corps, chasing out hundreds of State Department employees and scaling back the country’s engagement with the world. Most alarming has been the departure of dozens of the foreign service’s most senior officials—men and women who had spent their careers living and working abroad, who speak several languages, and who are experts in their fields. As I detailed in my recent Profile of Tillerson, he came into the job proposing to cut the State Department’s budget by a third, with plans to eliminate more than a thousand jobs and dramatically scale back the already measly sums America spends on refugees, democracy promotion, women’s rights, and the prevention of H.I.V. At the same time, the Trump Administration was proposing to dramatically increase spending on defense—by fifty-eight billion dollars, an amount that is larger than the State Department’s entire budget.

Tillerson’s proposed cuts were so galling that even some Republicans—Senators John McCain and Bob Corker, for instance—publicly protested. At a Senate hearing earlier this year, Corker told Tillerson that reading his proposed budget “was a total waste of time.” (In fact, the current Congress, so far, has been not been able to pass Trump’s budget.) In sum, Tillerson’s vision was of a vastly diminished role for America in the world, and a more militarized one.

As far as I could gather, Tillerson doesn’t have much of an ideology, apart from efficiency. As the C.E.O. of Exxon, Tillerson showed himself willing to make deals with any regime or any dictator, no matter how noxious the human-rights record or how corrupt, in order to secure more oil. He shared caviar with Vladimir Putin in New York, lobbied to undo sanctions against Iran, and set up subsidiaries that did business with Syria, Iran, and Sudan, whose regimes were all under American sanctions. When asked about these decisions, Tillerson did not seem much troubled by doing deals that were wildly at odds with his country’s foreign policy; Exxon, which operates in nearly as many countries as the State Department, was too important for that. “I’m not here to represent the United States government’s interest,’’ he told an audience in Texas while still at Exxon. “I’m not here to defend it, nor here to criticize it. That’s not what I do. I’m a businessman.”

It’s tempting to have some sympathy for Tillerson, given the boss he works for. President Trump seems to change his foreign policy daily, threatening war with North Korea one day, trashing NATO the next, or passing highly classified intelligence to Russian officials the day after that. When Tillerson travelled to China to try to search for a peaceful resolution to the nuclear crisis in North Korea, the President tweeted that Tillerson was “wasting his time.” “Save your energy, Rex. We’ll do what has to be done,” the President said. Shortly after his return, Tillerson, in a meeting with White House officials, was heard calling Trump a “fucking moron.”

Similarly, it’s tempting to buy into the popular notion that Tillerson (along with Secretary of Defense James Mattis; the President’s chief of staff, John Kelly; and the national-security adviser, H. R. McMaster) is one of the “adults in the room,” preventing the President from acting even more egregiously than he already has. What has Trump been prevented from doing? We don’t know what we don’t know, but the evidence is not encouraging. Tillerson has been humiliated by President Trump; even so, he has doggedly carried on with the President’s most radical policies.

Which brings us to Tillerson’s legacy. In the broadest sense, the world we live in was created by the United States. The architecture of international economic and political relations—the United Nations, NATO, the World Trade Organization, and so on—was largely drawn up by American diplomats at the end of the Second World War. The system they devised was meant to encourage the spread of free markets and liberal democracy, and it was premised, more than anything, on American leadership. It’s easy to trash the idea of American global leadership, imperfect and unjust as it has been. But what would the world be without it? Thanks in no small part to Secretary of State Rex Tillerson, we are about to find out.