“Today the guns are silent,” proclaimed General Douglas MacArthur from the decks of the battleship Missouri on September 2, 1945, after accepting Japan’s surrender in World War II. “A great tragedy has ended. A great victory has been won. The skies no longer rain death—the seas bear only commerce. Men everywhere walk upright in the sunlight. The entire world is quietly at peace. The holy mission has been completed.”

MacArthur merits top marks for waxing lyrical, and he was no slouch at generalship. But he hardly rates as an eagle-eyed observer of the postwar world. If the world rested quietly at peace, it did so only for an instant. Politics abhors a vacuum. War engulfed the erstwhile Japanese Empire anew as the surrender displaced Japanese colonial regimes across the region. Contenders warred to determine who would govern liberated lands from Malaya to Manchuria.

And Korea. Starting in the 1890s, Imperial Japan fought a series of limited wars to entrench itself in continental Asia. It annexed Korea and went to extravagant lengths to eradicate Korean nationhood. Bad blood continues to poison Korean attitudes toward Japan to this day despite the island state’s radical transformation. Indeed, to all appearances, Japan—not North Korea—stokes the most passion in South Korea today.

That’s tough for outsiders like yours to truly fathom. Japan has been a good international citizen for seventy years now, ever since U.S. forces ousted its militarist rulers in favor of a liberal republic displaying a strong pacifist streak. And for all the talk of Japanese rearmament, Tokyo spends a mere 1 percent of GDP on the armed forces. You can’t buy that much bang for so few bucks—certainly not enough to send forth conquering hordes across the Tsushima Strait.

Why fret over that pittance if you’re Korean?

Juxtapose inoffensive Japan against North Korea, a nuclear-armed totalitarian regime of dubious sanity that routinely menaces the South—at times with actual weapons fire. The Kim regime—the same one that invaded and nearly overran the South in 1950—remains in power, the leadership’s bombast undiminished. Pyongyang accepts neither South Korea’s political legitimacy nor its existence. A formal state of war endures to this day.

The Korean War claimed untold numbers of lives while leaving the peninsula in ruins. Pyongyang (and its Soviet and Communist Chinese backers) were directly responsible for the bloodletting. Southerners can clearly come to terms with past wrongs—as they have with the Korean War legacy. Their inability to do so vis-à-vis Japan to date carries major, and harmful, consequences for U.S. and allied strategy in the Far East.

Specifically, the diplomatic feud across the Tsushima Strait keeps the two Asian heavyweights from joining the United States in a tripartite marine alliance to police Northeast Asia. America is a close ally of South Korea. It’s a close ally of Japan. But seldom if ever do the two allies work together independently of the United States. That makes the U.S. Pacific Command the hub in a hub-and-spoke arrangement within the U.S. alliance system.

This is starkly suboptimal. Alliances thrive on mutual goals and strategy, and underperform when allies see one another—not external threats—as the problem. Adding a spoke connecting Seoul with Tokyo would do a world of good—but it would demand that they transcend the longstanding era of bad feelings.

Think about it. North Korea is a johnny-come-lately in Korean and world history. And however batty, it’s part of the larger Korean family. That takes some of the bite out of controversies embroiling Seoul and Pyongyang. Korean-Japanese enmity, by contrast, dates back further than World War II. It goes back further than 1910, when Japan annexed the peninsula. It goes back even further than 1894-1895, when Imperial Japan launched into its continental misadventures during the First Sino-Japanese War.

Indeed, Japan’s record of mayhem stretches all the way back to the sixteenth century while spanning the military and, after 1868, imperial regimes. Koreans may well fear that the last seventy years represents the calm before the next storm. Liberal rule today, militarist rule tomorrow?

During the Imjin War (1592-1598), hegemon Hideyoshi Toyotomi—who had recently unified Japan through a string of operations against the daimyo, or regional warlords—dispatched a massive expeditionary army to the Korean Peninsula on an errand of conquest. Hideyoshi professed megalomaniacal aims. Crushing Korea was only the beginning. He wanted to oust the Ming Dynasty and enthrone a Japanese emperor in China. He demanded tribute from the Philippines’ Spanish rulers. He even entertained the notion of bringing India under Japan’s yoke.

Call it the Greater East Asian Co-Prosperity Sphere, sixteenth-century style. The Imjin War is replete with insights into the nature of diplomacy and warfare and the passions and calculations that impel human endeavors. It also has much to say about the peculiarities of fighting on and around the Korean Peninsula, in effect a half-island grafted onto continental Asia. Hideyoshi’s army landed in 1592 and drove swiftly northward, taking Seoul and Pyongyang before the offensive sputtered to a halt and went into reverse.

Why did it sputter? Well, the Korean defenders adapted to superior tactics and hardware, as defenders often do. Koreans proved inferior to Japanese soldiers in open-field combat, yet it turned out they excelled at defending compact fixed fortifications. And Korea had powerful friends. Having fled the capital, the Korean king Sonjo beseeched the Ming emperor Wanli to intervene. A modest-sized Ming army marched onto the peninsula, adding China’s weight to the balance.

The Korean navy, meanwhile, acquitted itself brilliantly under the tutelage of Admiral Yi Sun-sin, one of naval history’s all-time badasses. (At the climactic sea engagement late in the conflict, he took on a fleet of over 200 Japanese vessels with a flotilla of 13 ships—and won. No word on whether the makers of 300 plan an East Asian edition of their saga of martial derring-do.)

Equipped with sturdily built “turtle ships” and “board-roofed ships” festooned with cannon, Yi’s fleet scored a series of bracing tactical victories off the southern coast. Ultimately the Koreans denied the Japanese navy the use of the waters off the peninsula—and thus kept Japanese commanders from resupplying ground forces to the north by sea. The allies starved the Japanese contingents while evening the military balance on the peninsula.

Between Chinese intervention and Korean command of the sea, then, the Japanese expedition stagnated. Japan clung only to southern enclaves. A second invasion late in the war accomplished little. Japan gained nothing from the enterprise—and slunk home following Hideyoshi’s death in 1598.

And the implications of this long-ago episode for the present state and prospective future of Korea-Japan relations? Look at it through Korean eyes. One, there’s a pattern to Japanese conduct that extends back over four centuries. It’s hard not to harbor anxiety that Tokyo might someday yield to the imperial temptation once again. Two, Japanese forces can operate on the peninsula and do cataclysmic damage—even if repulsed in the end. And three, courting strong allies while commanding offshore waters constitutes a prudent hedge against a resurgent Japan.

This is not to argue that Tokyo must apologize more fervently for World War II—let alone for the Imjin War—or that Washington must prod Tokyo to adopt any particular measures to foster concord within the alliance system. Nevertheless, hedging within an alliance rather than making common cause is a troublesome thing. If knowing the enemy and knowing yourself charts the route to triumph, surely knowing your ally is part of self-knowledge. Familiarity—and a measure of empathy with how Koreans see their surroundings—could work wonders for alliance management in Northeast Asia.

One imagines an old Pacific soldier like Douglas MacArthur would go along with that.

James Holmes is Professor of Strategy at the Naval War College. The views voiced here are his alone.