Ranking battles by their importance has been a bloodsport among military historians as long as there have been military historians. Creasy's classic Fifteen Decisive Battles of the World(1851) set the standard for the genre.

But what makes a battle decisive? And what makes one such test of arms more important than another?

Defining the term too loosely produces howlers like a recent US News catalogue of decisive battles of the American Civil War. By the US News count, 45 engagements qualified as decisive during that four-year struggle alone. Zounds!

The term must be defined less cavalierly than that to be meaningful. If every battle is decisive, no battle is. That's one reason I always ask students whether some legendary triumph—a Trafalgar, or a Tsushima Strait—was decisive, or just dramatic, or just featured the star power of a Nelson or Togo.

Even the masters of strategy, however, appear ambivalent about what constitutes decisive victory. Carl von Clausewitz supplies a working definition, describing a decisive engagement as one that leads directly to peace. This implies an action carrying not just tactical but strategic and political import. Such an encounter impels the vanquished to accept the victor's terms at the bargaining table, whether because he's no longer capable of fighting on, believes he stands little chance of turning the tables and winning, or estimates that victory will prove unaffordable. A decisive battle, in this expansive interpretation, is the chief determinant of a war's outcome.

So far, so good. But how direct must direct be for a battle to earn the lofty status of decisive? Must peace talks take place immediately following the clash of arms that precipitates them? Can a settlement come weeks or months afterward, so long as the cause/effect relationship is clear? What's the time horizon?

The great Carl is silent on such matters. It's possible he's too casual about them. Combat, methinks, can be decisive while achieving more modest goals than winning a war outright. To see how, interpret the word literally: something decisive decides something. (Admittedly, this is probably how the US News team got in trouble. Every action decides something in a tactical sense, no matter how mundane or inconsequential. If nothing else, it determines who holds the field of battle at day's end, or reveals that the fighting stalemated. This says little about its larger meaning, if any.) Armed clashes can yield decisive results on different levels of war. A tactical encounter could decide the outcome of a campaign or the fate of a combat theater without leading directly to peace. Right?

In short, it appears wise to define a decisive victory more three-dimensionally than Clausewitz does, namely as a trial of arms that lets a belligerent accomplish some positive or negative aim beyond mere tactical results. Winning the war would still qualify, obviously, but the broader view would allow historians to rate a Battle of Trafalgar as decisive.

Fought in 1805, Trafalgar scarcely brought about final peace with Napoleonic France. That took another decade of apocalyptic warfare. But it did settle whether the French could invade the British Isles and, through amphibious conquest, crush the offshore threat to French supremacy. The heroics of Nelson, Collingwood, and their shipmates decided the outcome of Napoleon's scheme while enabling Great Britain to persevere with the struggle. Trafalgar, then, directly accomplished the negative goal of keeping French legions from invading Britain. That must qualify as decisive in the operational sense.

Speaking of pugilism on the briny main, here's another wrinkle in this debate. Clausewitz says next to nothing about maritime conflict. Water barely exists in his writings. Fin de siecle historian Sir Julian Corbett, the best in the business of sea-power theory, doubts naval warfare is ever decisive by itself, except perhaps through gradual exhaustion. Close or distant blockades, however, grind down not just the enemy but your allies and your own businessfolk who rely on seaborne trade. They impose costs on everyone.

Like Clausewitz, Corbett thus seems skeptical about the decisiveness of any single engagement. Sure, a dominant seafaring state can and must make the sea a barrier to invasion and other direct assaults on its homeland. That's the Trafalgar model. At its bottom, though, maritime strategy is the art of determining the relations between the army and navy in a plan of war. It's ultimately about shaping affairs on land, which, after all, is where people live. But how do you rank a purely naval engagement on the high seas against an army/navy operation that unfolds at the interface between land and sea?

Tough question. So there's a lot to ponder in the seemingly simple question, what is decisive? To rank naval battles against one another, let's assign a pecking order among degrees of decisiveness. Derring-do, tactical artistry, or gee-whiz technology are not among the criteria. Nor are lopsided tactical results enough to lift an encounter into the upper echelon. That's why the Battle of Tsushima, which left wreckage from the Russian Baltic Fleet littering the Yellow Sea floor in 1905, doesn't make my list.

Topping my list are naval actions that decided the fates of civilizations, empires, or great nations. This is decisiveness of the first order. Such encounters stand in a category apart. Next come engagements that reversed the momentum in a conflict of world-historical importance. They set the endgame in motion, even if a long time elapsed between the battle and the peace settlement it helped set in motion. Direct needn't mean fast. And if battles meeting one of those standards don't exhaust the field—read on to find out—last come tests of arms that settled the outcome in a particular theater of war, helping set the stage for eventual victorious peace.

So, this leaves us with a rough hierarchy of sea fights. The more fateful and enduring a battle's ramifications, the higher it stands on the list. All of which is a roundabout way of getting to my Top Five Naval Battles in World History. In order from least to most important:

5. Lepanto (1571). The Battle of Lepanto stemmed the westward spread of Ottoman power across the Mediterranean Sea. With papal sanction, the Holy League, a consortium of Catholic seafaring states, assembled a navy to engage the main Ottoman fleet in the Gulf of Corinth, off western Greece. Lepanto was the last major all-galley naval engagement in the Mediterranean. The League put its advantages in gunnery and ship types—notably the galleass, an outsized galley bearing heavy armament—to good use against the Turkish fleet, which disgorged far less weight of shot. The Ottomans lost the bulk of their vessels to enemy action. More importantly, they lost experienced crews. They found that regenerating human capital isn't as easy as fitting out wooden men-of-war. Shipwrights soon built new hulls, letting the Turkish navy reassert its supremacy in the Eastern Mediterranean for a period. By 1580, however, the new navy was left to rot at its moorings. Galley warfare assured permanent European, not Ottoman, command of the middle sea. That's a decisive result by any measure.

4. Battle of Yamen (1279). Sometimes dubbed “China’s Trafalgar,” this clash between the Mongol Yuan Dynasty and the beleaguered Southern Song determined who would rule China for nearly a century. It was far more decisive than Horatio Nelson’s masterwork. Over one thousand warships crewed by tens of thousands of men took part in the engagement. Yuan commanders deployed deception and bold tactics to overcome at least a 10:1 mismatch in numbers. Most important, Yamen claimed the life of the Song emperor, clearing the way for Kublai Khan’s dynasty to take charge of Asia's central kingdom. The results of Yamen thus reverberated throughout Asia for decades afterward.

3. Quiberon Bay (1759). The year 1759 has been called a year of miracles for Great Britain. Land and naval operations determined whether Britain or France would emerge triumphant in North America. British troops under Wolfe subdued Montcalm's defenders at Quebec and Montreal, sealing the fate of New France. France stood little chance of recouping its fortunes because Admiral Sir Edward Hawke's Royal Navy fleet ventured into Quiberon Bay that November—in a westerly gale, no less—and put paid to the French fleet in its home waters. Having wrested away command of Atlantic shipping lanes, Britain could bar access to the Americas. Lesson: to score decisive victories, hire commanders with fighting names like Wolfe and Hawke. British exploits settled the destiny of a continent while setting a pattern for North American politics that persists to this day.

2. Spanish Armada (1588). This was the duke of Medina Sidonia's purportedly invincible fleet, ordered by Spain's King Philip II to cross the Channel and land in England. There Spanish forces would unseat England's Queen Elizabeth I. By installing a friendly regime, the expeditionary force would terminate English support for the Dutch revolt roiling the Spanish Netherlands while ending English privateering against Spanish shipping. The Catholic Philip sought and obtained papal approval for the enterprise against the Protestant Elizabeth. Weather, however, conspired with English seamanship and gunnery tactics to condemn the expedition. The Spanish host was unable to land. Instead Medina Sidonia found himself compelled to circumnavigate the British Isles under foul conditions to reach home port. The failed cross-Channel crusade heartened the English crown. Had the Armada replaced a Protestant with a Catholic monarch, it's conceivable that the British Empire never would have been founded—and certainly not in the form it actually took. How would Atlantic, Pacific, and Indian Ocean history have unfolded then? The implications of that question boggle the mind—and qualify the Armada's defeat as decisive in the largest sense.