There are several compelling reasons not to combine the jobs of national security adviser and secretary of state. Let’s boil them down to these: history, alliances and prior successes and failures. Tragically, when it comes to managing our national security, lessons learned appear not to factor into President Trump’s calculus.

Prior service on the National Security Council staff under the stewardships of Henry Kissinger and Brent Scowcroft give me concern that Mr. Trump followed his justifiable ouster of Michael Waltz as national security adviser with the unjustifiable move to add that role to Secretary of State Rubio’s portfolio.

These job adjustments are significant and doomed to fail. They are also less a cause of chaos than a reflection.

Trump’s stewardship of our external relations as he pulls the pins out of the world order the U.S. was instrumental in creating after World War II invokes cold fear.

The swift rift between Trump and Waltz was avoidable if either had any concept of national security strategy, how the National Security Council system has been managed or how a president is best served by the national security adviser. A president and the national security adviser must have a bonded relationship based on substance, procedure and the adviser’s backbone and expertise to offer unvarnished, perhaps unwelcome advice. All these were lacking with Waltz.

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Now we are hardly better off, because being NSC adviser and secretary of state are each more demanding jobs than most mortals can handle.

The national security adviser is a White House function that distills and serves up recommendations from the executive branch to the president. The advisor sees the president first thing in the morning for the daily intelligence briefing and leaves after the president retires for the evening, but remains on call around the clock. Barring unforeseen crises, Brent Scowcroft was at the White House before 8:00 a.m. and left after 10:00 p.m., with somewhat less demanding schedules on Saturday and Sunday.

The adviser needs to anticipate the president’s requests and preferences and keep NSC member agencies informed. He or she requires the credibility both to reflect the president and to think independently. I hope Mr. Rubio understands that the function will be especially difficult when national security decisions are not made based on the wealth of perspectives from NSC members, but rather on the president’s brainstorms and sometimes disdain for his own agencies.

The secretary of state’s management role is different from that of the NSC adviser and also grueling. Beyond overseeing the sprawling State Department, the secretary is America’s face and voice to the world, drawing support from and reassuring allies (back when such cooperation was prized), keeping channels open with adversaries to avoid unnecessary conflict, and working to win support from virtually every other nation. The job requires nuanced negotiation and diplomacy and, of course, relentless travel.

Substantively, there are some similar, required skill sets but also clear distinctions between the two jobs. The NSC adviser has to understand diplomacy, intelligence, economics and defense to be trusted by NSC members to convey their perspectives. The adviser must also demand coherent excellence and strategic consistency in the NSC’s submissions to the president and be able to distinguish between a problem and a crisis.

The secretary must be a foreign policy expert who commands the ressect of counterparts around the world. A key, highly specialized requirement is to judge the potential of diplomacy to mitigate all national security challenges.

It’s a heavy lift for former Sen. Marco Rubio to succeed in either job, much less to combine them, even temporarily. His international background is not deep, and he suffers the awkwardness of defending Mr. Trump’s views which until recently were contrary to his own. Managing both the procedure and substance of Trump’s national security agenda is all the more fraught, given that it often depends on the president’s simplistic instincts about the world and his business interests.

The combination was actually attempted once before, when President Nixon made NSC Adviser Kissinger concurrently secretary of state. But Rubio is no Henry Kissinger; it’s telling that this arrangement never occurred again, until now. The same centrifugal forces tearing at the combination still pertain.

Kissinger became secretary after he had fatally weakened Secretary of State William Rogers and had the luxury of Brent Scowcroft being his NSC deputy. Scowcroft ran the NSC from the White House and is widely credited with later becoming the best NSC adviser the country has ever had.

The experiment failed because Nixon’s successor, President Gerald Ford, saw that the country needed the full-time dedication of a skilled professional in each position. There was also blatant conflict. When Kissinger convened a meeting of NSC principles, was he NSC adviser (seeking to pull the best from the many perspectives at the table) – or the secretary (advocating for his own department)?

Managing national security affairs is not a detached experiment. It is as delicate a minefield as one could imagine. Trump’s convoluted relationship with Russia’s President Putin, the Ukraine war, a looming tariff war, and China’s delight that we are doing what they have sought for decades – causing a split in the West – all demand attention. Then there is the Middle East, Iran and a catalog of other concerns.

We need a coherent NSC system that draws forth the best intelligence from our talented Intelligence Community, wisdom from our seasoned military and the acumen of skilled diplomats.

We need a State Department leader who can focus on traveling the globe, parsing the powers of soft and hard diplomacy and making complex decisions, while juggling many balls in the air at the same time.

Instead, we witness the president’s disregard, perhaps ignorance of our NSC by asking Secretary Rubio to take on these two jobs.

A final note. Meddling with the component pieces of our national security operation is not even the main problem.

Rather, our attention and fear should be directed toward Mr. Trump’s destruction of the world order created after World War II with no apparent idea of what will replace it. Aggression and chaos fill vacuums. We face that prospect with a muddled NSC system in place, an overextended secretary of state and a president pleased with his own instincts to protect our future.

Arthur House, of Simsbury, was special projects officer for Henry Kissinger and Brent Scowcroft at the National Security Council. He is an adjunct professor at the University of Connecticut.