In this powerful nation with founding principles and latent capacities second to none, politics have become fit for the fall of Rome, the culture is sick with self-destruction, and the rule of law is routinely perverted. Though politics, culture, and law are the arch of the nation, the keystone without which they cannot hold is defense. For war transforms whole peoples and threatens their sovereignty and national existence more decisively than any other force.

You would hardly know this from the current presidential campaign. Most of the candidates seem unaware that the prospects of catastrophic war in the not-so-distant future are burgeoning because of a fundamental change in the international system, driven by accelerating adjustments in relative military power.

Russia, China, and Iran have been racing ahead, stimulated by a disintegrating Europe that neither spends sufficiently on its defense nor defends its borders; and by a United States, strategically blind in the Middle East, that failed to replenish and keep current its military under President George W. Bush, and now surrenders, apologizes, bluffs, “leads from behind,” and denigrates its military capacities and morale as President Obama either embraces enemies or opposes them only with exquisite delicacy.

As the U.S. allows its nuclear forces to stagnate and decay into de facto unilateral disarmament, Russia has been modernizing its own. The Kremlin has added systems, such as road-mobile, intercontinental ballistic missiles with multiple independently targetable warheads, that we neither have nor envision. In the absence of “soft power” parity with the U.S., Russia dangerously relies on a permissive nuclear doctrine and promiscuously rattles its atomic sabers. Its nuclear adventurism, naval and land force modernization, unopposed reintroduction into the Middle East, invasion and annexation in Ukraine, and the ability to recapture the Baltic states in an afternoon are yet another impeachment of “the end of history.”

With little resistance, China incrementally annexes the South China Sea while embarked on a naval buildup inversely proportional to the smallest U.S. fleet since 1916, and further aggravated by China’s ability, once its naval technology matures, to surge production in its 106 major shipyards as opposed to America’s six. More importantly, China is expanding its nuclear forces—to what extent we do not know, because the Chinese program’s infrastructure is hidden within 3,000 miles of tunnels largely opaque to U.S. intelligence. As if China were not a major rival, the Obama Administration, ever infatuated with accords, has made no effort to include Beijing in a nuclear-arms-control regime. Why not?

We pay Iran for allowing us to stabilize its acquisition of nuclear armaments, and discount both the North Korean threat and missile defense, the only means of opposing it. As North Korea and Iran work up to minimal nuclear capacity, this administration works down to it, thus (in a mistaken conception of nuclear sufficiency) encouraging proliferation and eventual parity among a large number of nuclear states. Nothing could be more dangerous.

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Should these trends continue unaddressed, the world will see three great powers—China, Russia, the U.S.—each with a complex and shifting system of alliances in unstable areas subject to proxy wars and opportunistic territorial expansion, the beginnings of which are now apparent in Ukraine, Syria and the South China Sea. As Wilhelmine Germany was either unwilling or unable to restrain Austria from invading Serbia despite the latter’s abject submission, thus precipitating World War I, no single power will be able effectively to discipline its allies.

With rapid shifts in the correlation of forces among near equals, nations seeking protection will migrate among the blocs and arm to protect themselves, provoking their neighbors to do the same. Such conditions, absent since World War II, will be remarkably unstable, especially given the emergence of semi-medieval crazy-states armed with nuclear warheads on ICBMs. To prevent or weather these dangers, the next Congress and administration must rearm America and insist that our allies follow suit. The American nuclear deterrent must be refreshed and augmented. China must be brought into a nuclear-arms-control regime. The U.S. and its allies must take a much harder line and accept greater risks to halt proliferation, starting with our greatest failure to date, Iran.

In Europe, U.S. forces in divisional strengths must combine with similar British, French, and German formations to deploy in France and Germany, like a movable piston, subject to advance or retreat governed by either the provocative or reasonable behavior of Russia. The varying rotation and basing of detachments on the periphery would work as a similar means of signal and deterrence. We send weak forces eastward now as a message, but behind them is virtually nothing of force or will.

In combination with Japan, India, Vietnam, and the Philippines, newly armed with long-range antiaircraft and coastal-defense missiles, an American fleet at least half again its present size, and the F-22 tactical fighter brought back from the dead, can frustrate Chinese claims to the South China Sea. Those claims are as much of a portent and even less legitimate than the Third Reich’s claim to the Sudetenland.

The U.S. 2015 base budget defense appropriation (excluding overseas contingency funding) was just less than 3%, as opposed to 5.7% in the peacetime years during the period from 1940 to 2000. Though, embarrassingly, it would take Churchillian statesmanship to return merely to the norm, doing so would help arrest the slide toward a perilous international system such as existed before World War I. But in a presidential campaign in which most candidates and their partisans see only what is at their feet, perhaps it is too much to expect that they look toward the horizon.

An earlier version of this essay appeared in the Wall Street Journal.