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.