There will be many reasons to miss Secretary of Defense Jim Mattis. His experience in combat, camaraderie with troops, calm in crisis management and credibility (until very recently) with President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE have all been huge advantages for the nation.

He has often been called the adult in the room — when one imagines what many of the conversations in the Situation Room of the White House have been like these last two years.

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But I will miss Mattis even more for being the adult in a city that risks losing its collective mind over competition with Russia and China. Some talk of a new cold war; many in the national security community seem to think war against one or both is not only possible, but likely.

These individuals are too quick to forget the timeless words of Bernard Brodie, one of the country's top nuclear strategists, who wrote soon after World War II that until now, the purpose of the nation's military forces had been to win major wars, and now it must be to prevent them. The distinction is crucial among nuclear-armed superpowers.

Mattis has, to be sure, been as fixated as anyone on the threat posed by Russia's return and China's rise. His 2018 National Defense Strategy, which will surely be among his most important legacies, emphasizes the return to great-power competition and the priority to which the United States and allies must now accord efforts to deter these two great powers. All that is right and appropriate.

But Mattis undertook the task with a certain calm. He did not see every islet in the East China Sea or South China Sea, every unfortunate incident in eastern Europe, every Russian machination in Syria as tantamount to the opening salvo in World War III. Rather, his approach has been businesslike: firm, resolute, but non-escalatory and calm.

For example, back in January 2017, when Secretary of State-designate Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE told the Senate that the United States must prevent China from further access to the islands it had reclaimed in the South China Sea — a proposal that sounded reasonable until you realized that it could easily lead to great-power war over uninhabited pieces of rock — Mattis followed up with wise words to the effect that the issue could be handled diplomatically.

To be sure, he sent a few aircraft carriers to the region over the years, believing that military strength could and would aid diplomacy. But he never saw the military instrument as the principal way of solving such problems.

Mattis's successor will need to emulate this kind of nuance, patience and wisdom. We do need to reinvigorate our defenses and our resoluteness against a whole host of possible aggressions and assertions by Moscow and Beijing.

But we are in a strong enough position, with enough military power and enough allies, that we must avoid overreacting or thinking that the world is about to collapse because China and Russia are gnawing around its edges and that there are echoes of Hitler's ambitions in President Xi and President Putin.

A month after the world has commemorated the end of World War I — a horrible war that Mattis, with his passion for history, studied and understood — we need to bear in mind the possibility that competitive great powers can produce cycles of insecurity, rivalry and provocation that lead to a war no one would have wanted.

That kind of danger, in today's world, is at least as real as the fear that an insatiable aggressor nation could keep expanding its appetite and ambitions the way that Nazi Germany or Tojo Japan did in the 1930s and 1940s.

In other words, mutual overreaction to small crises can produce war just as easily as weakness and deterrence failure can.

Mattis has had the wisdom to remember both world wars, and both possible paths to war — appeasement of a dangerous aggressor, on the one hand, and rivalry among great powers producing a spiraling cycle to conflict on the other.

He has been the adult in the room, and the adult in national security circles writ large. We need a successor with the same judicious, thoughtful and wise view of history — and of today's dangerous moment.

Michael O'Hanlon is a senior fellow and director of research of the Foreign Policy Program at The Brookings Institution.