U.S. military forces planned to ultimately seize Canada’s rich mineral resources around Sudbury, Ontario, but in the interim, they would launch surprise attacks on the key ports of Halifax, Nova Scotia, and Vancouver. While many planners disliked the prospect of using poison gas on civilians, it was essential to knock out those strategic centers swiftly, before the British mounted their inevitable counter-offensive. The British amphibious invasion would most likely land between Ocean City, Md., and Rehoboth Beach, Del.

If the above sounds like a Harry Turtledove alternative history scenario, it isn’t. What I have just described is the War Plan Red developed by the U.S. armed forces in the late 1920s, which remained on the books as a formal contingency plan through 1939. Such planning—and many other imaginary scenarios developed before it—reminds us of how frequently the U.S. and other nations have identified particular foes who would certainly demand to be defeated and destroyed. Often has the refrain gone: if war with such an enemy is utterly inevitable, why not start it now, and get it over with?

A little historical perspective, though, should make us quite humble about the whole notion of “inevitability,” not to mention the prospect of perpetual enemies.

Two centuries ago, in 1815, the British Empire was fighting two key enemies, respectively: France’s Emperor Napoleon and the new United States of America. Although Britain and the U.S. formally concluded hostilities the previous year, the culminating Battle of New Orleans did not occur until the start of 1815. The decisive British victory at Waterloo followed in June.

Had you told any informed observer in 1815 that Britain would never again engage in formal hostilities against either of those nations—not even in two entire centuries—you would have probably have been labeled as insane. Rivalry with the French had been the absolutely consistent factor in British affairs since 1689, and was clearly not going to vanish overnight. As historian Jules Michelet sagely remarked later in the 19th century, in understanding world affairs, there is France and Britain, and that is all.

For the rest of the 19th century, the question of the next Anglo-French war would be when, not if. Tensions reached ugly heights in the late 1850s, immediately after the Anglo-French cooperation in the Crimean War. During the Indian Mutiny of 1857, patriotic French newspapers suggested immediately sending the fleet to help the rebels evict the British Raj. Britain suffered a full-scale invasion scare in 1859-60, when volunteer rifle companies sprang up across the country to resist the threat of French occupation. The two countries came within an ace of open warfare in 1898, over colonial rivalries on the Nile.

Somehow, though, the inevitable struggle never occurred. One can legitimately point out that the two armed forces were at war in the 1940s, when the British fought to evict the Vichy French from colonial territories like Syria and Madagascar. Technically, though, this fell short of actual declared warfare, and the resulting battles have slipped into the realm of polite oblivion.

But even if the French menace could somehow be contained, surely the next American war was truly only a matter of time.

In Herman Melville’s books, especially White-Jacket (1850), we repeatedly are given the sense that conflict on the seas was imminent and inevitable. Naval and imperial rivalries made warfare certain, as did U.S.-Canadian border rivalries: frontier disputes in the Oregon territory stirred American war fever in the 1840s. Between 1840 and 1900, serious war scares were running at about one per decade.

At the start of the Civil War, heavy-handed U.S. actions against Confederate envoys on the high seas almost brought the British into the struggle. The U.S. owes an immense debt to Prince Albert, who drafted the diplomatic documents that kept the British out of war, and presumably saved the American Union. Time and again, from the 1860s onwards, border-crossing activities by Irish guerrillas threatened to sabotage the fragile U.S.-British modus vivendi. The two countries once more came close to war over Venezuela’s borders in the 1890s.

And that brings us to the 1920s, the era of War Plan Red. To U.S. commanders observing the likely military future at that time, by far the deadliest danger they could foresee was a British-Japanese alliance that would overwhelm the U.S. Navy. Canada, tragically, would be the battleground between the two great English-speaking nations, those inevitable foes destined to fight until only one survived. Who can withstand destiny?

Needless to say, none of those nightmare scenarios ever came to fruition. Somehow, the British evolved from being our eternal foes to becoming those nice folk across the pond who send us Masterpiece Theatre and Benedict Cumberbatch.

Other seemingly inevitable crises likewise failed to materialize.

People of the boomer generation might remember the hyperventilated coverage of Chinese events during the Cultural Revolution of the post-1966 decade. According to most reports, the country had seemingly created a generation of tens of millions of crazed fanatics pledged to world conquest. How could they be stopped, short of a nuclear pre-emptive strike (which the Russians were actually contemplating in 1969)?

The Cultural Revolution was indeed a ghastly tragedy for the Chinese people, but the feared external aggressions never occurred. By the late 1970s, the main organizers of that fanaticism were themselves discredited and imprisoned, and China was ready to rejoin the world community. Again, the “inevitable” proved to be an illusion. Somehow, too, the “unavoidable” global clash between the U.S. and the Soviet Union was, well, avoided.

We can debate the best means of preventing wars, and sometimes, a strong and vigilant military might indeed be the best way of keeping the peace. But if anyone ever says today that conflict with a particular nation or cause is “inevitable,” whether that contemporary foe is Iran, China or Russia, history offers plenty of reasons to doubt such claims. Somewhere down the road, in fact, those adversaries might become our best friends. Never say never.

Or to quote Lord Palmerston, speaking of England: We have no eternal allies, and we have no perpetual enemies.

Philip Jenkins is the author of Images of Terror: What We Can and Can’t Know About Terrorism. He is distinguished professor of history at Baylor University and serves as co-director for the Program on Historical Studies of Religion in the Institute for Studies of Religion.