Back in 2013, Gen. James Mattis warned Congress about the dangers of undercutting U.S. diplomacy.

“If you don’t fund the State Department fully, then I need to buy more ammunition,” said the general, who was then head of U.S. Central Command. Mattis recognized that skilled diplomacy is required to prevent wars or cement military victories.

Fast forward to now, when President Donald Trump not only wants to slash the State Department’s budget by a third but clearly considers the department unnecessary. His vision of foreign policy revolves around military strikes and arranging mythical “big deals” - while tweeting bizarre missives at world leaders.

Meantime, Secretary of State Rex Tillerson conducts diplomacy as if he were still the god-head of Exxon Mobil, consulting only a few aides while facilitating the gutting of the department. Most key posts and many crucial ambassadorships remain vacant.

At a time when skilled U.S. diplomacy is desperately needed to deal with North Korea, Iran, Afghanistan and Syria, Trump and Tillerson seem bent on dismantling our ability to act.

The big disappointment is Tillerson, whom many (myself included) had hoped would bring his global experience as an oilman to his new job. The expectation was that he would join a talented troika (with Defense Secretary Mattis and National Security Adviser H.R. McMaster) to temper Trump’s worst foreign policy impulses.

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Didn’t happen.

“Tillerson is as big a problem as Trump is,” says a former U.S. ambassador to Yemen, Gerald Feierstein, now director of gulf affairs at the Middle East Institute. “He has no respect for the mission of the State Department. He’s distant, disinterested in having any interaction with employees.”

The oilman has displayed the same disdain for American diplomats as his boss, failing to protest the size of the proposed cutbacks; he is conducting a “reorganization” that will paralyze the department until mid-2018.

Only three nominations have been put forward to permanently fill 22 assistant secretary positions, which include the top department posts for critical regions like the Near East, East Asia and Afghanistan. Instead, these positions are being filled by “acting” heads, whose clout is limited by their temporary status. The suspicion among diplomats is that the White House wants most positions filled with political appointees, not department professionals.

The posts of special envoys for climate change, for conflict and stabilization operations, and for nuclear nonproliferation (among many others) remain vacant, with the suspicion they will never be filled.

At the same time, no nominations have been put forward for dozens of key ambassadorships, including to critical countries such as South Korea. Nor are current ambassadors spared the disdain for their profession that Trump and Tillerson appear to share.

The secretary has become notorious for his reluctance to meet with U.S. ambassadors in the countries he’s visited or even when they return home to accompany foreign leaders visiting the United States.

As a consequence, morale at the State Department is sagging, and many of the most talented diplomats are quitting or retiring. Such expertise takes decades to develop and now is being thrown away.

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Moreover, there is another, less noticed, consequence of denuding the department of its talent. Foreign officials and ambassadors in Washington are realizing it’s pointless to speak to “acting” heads of departments or U.S. ambassadors. Many decide that their only avenue is to try to curry favor directly with the White House, ideally with First Son-in-Law Jared Kushner, as if this were a banana republic.

Some diplomats to whom I’ve spoken excuse Tillerson because they think he’s still trying to find his footing with the White House. He has had his own problems with Trump, who publicly undercut the secretary’s efforts to patch up a messy confrontation between Saudi Arabia and Qatar (the confrontation was actually encouraged by Trump). And the president has outsourced key Mideast tasks to Kushner, who has proven to be woefully unsuited and way in over his head.

But the president’s fascination with things military ignores the fact that America’s most immediate challenges demand diplomatic solutions if war is to be prevented. That includes resolving the impasse with North Korea, re-examining the nuclear deal with Tehran and negotiating over how to end the sectarian conflict in Syria that spawned the Islamic State.

Such diplomacy can’t be finessed by a real estate mogul or a lone oilman, no matter their illusions. It will require input from professionals with on-the-ground experience who can help Tillerson develop a policy process along with negotiating terms.

That, in turn, requires a secretary of state who understands he can’t act solo, and must encourage the best and brightest in his department and stand up for its survival. It also requires a secretary who is willing to stand up to Trump, or join in the aforementioned troika that may be able to penetrate the president’s tweet-drenched delusions.

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Tillerson should study Mattis’ 2013 warning to legislators to view diplomacy in terms of a cost-benefit ratio. “The more that we put into the State Department’s diplomacy,” the general said, “the less we have to put into a military budget as we deal with the outcome of an apparent American withdrawal from the international scene.”

Although Trump will deny it, that warning is even more relevant today. The denigration of diplomats is viewed abroad as a sign of a new U.S. isolationism and a signal for adversaries to take advantage. If Tillerson believes that to be an incorrect message, he’d better change course.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.