When did America’s mythic push West get under way? When did we become an imperial nation? These sorts of questions have transfixed historians for generations. Such questions have also sent historians into fits. Did expansion start from the get-go with the Puritans who sought to build a “city upon a hill” by “improving the land?” Or did it begin with the settlers in Virginia who fought with Indians over the course of the 17th century? Or did conquest as a defining ideology take shape before English settlers ever set foot in America? Perhaps we should consider specific watershed moments, such as 1676 with the end of King Philip’s War and Bacon’s Rebellion. In both episodes, colonists slaughtered Indians with a racist fury. Or perhaps the answer lies in the 18th century with the Seven Years’ War, after which the French as a countervailing presence left North America? Or is it the American Revolution, when the newly independent United States won territories stretching from the Atlantic to the Mississippi? Or the Louisiana Purchase? All these events are likely candidates, and scholars have spilt a great deal of ink championing some of these at the expense of others.

Equally challenging is the question of why expansion happened. We have all sorts of explanations for this question. At one extreme, some argue, as did the historian Frederick Jackson Turner more than a century ago, that the nature of the frontier spurred expansion. Pushing forward a line separating savagery and civility created a democratic ethos and the compulsion to spread civility further. At the other end of the spectrum, others argue that some essential European trait such as restlessness, the desire to be free, greed, or racism — what historians once characterized as a European “germ” that could overpower the American environment — explains why Americans would look to the West.

Figuring who bears the ultimate responsibility for western expansion can also prove exacerbating. At the most basic level, we need to ask if common men and women, looking to achieve competency, were to blame, or if visionaries such as Thomas Jefferson who dreamed of a western “empire of liberty” lay behind the impulse. The how of western expansion is much simpler to understand. Settlers would displace Indians with all kinds of justifications. Disease, displacement, dependency, and the vicissitudes of warfare transformed Indians from forces to be reckoned with to victims of white avarice.

So we know a great deal of who, what, when, where, how, and why, but we still debate the issue. We return again and again to the tangled relationship between westward expansion and America’s historical identity because such sentiments still go to the heart of who we are, as well as — for better or worse — our notions of exceptionalism. Coming to grips with America’s expansiveness matters now more than ever. Especially in a time when the people wonder what their government is doing in their name in distant lands, these represent a significant set of issues that should engage us all.

Richard Kluger, a Pulitzer Prize winner and author of a marvelous book on the history of Brown v. Board of Education, now enters the fray with “Seizing Destiny: How America Grew from Sea to Shining Sea” (Knopf, 672 pages, $35). This is a massive tome, offering a reading, or better a litany, of the events, pressures, and dynamics that led Americans to seize a continent, and even to dream of super-continental conquest. Mr. Kluger boils down a complex set of processes to this: Europeans, particularly Englishmen and women, freed by the Reformation and the Enlightenment from feudal strictures to think of themselves as individuals became storm troopers for British imperial, and later American national territorial expansion.

For Mr. Kluger, this impulse of personal independence impelled westward movement, and great proponents of expansion, such as Benjamin Franklin and Jefferson, amplified the inevitable. The availability of land, and the willingness to ignore Indian claims to it, allowed this sensibility to flourish. From the earliest Elizabethan sea dogs like Walter Raleigh, and the poor men that sailed with them, to the troops sent down to topple Manuel Noriega, and all points in between, Americans felt justified in becoming lords of all they surveyed. In blow-by-blow fashion, restless settlers conquered Virginia and New England in the 17th century, the Ohio Valley in the 18th, and the Louisiana Territory and parts of Mexico in the 19th.

Expansion for Mr. Kluger has an irresistible quality. Although he covers a great deal of diplomatic wrangling and saber-rattling that went into creating an imperial nation, he does not see this as a history of watersheds or critical turning points. Mr. Kluger makes a plea for simplicity. America’s common conquistadores and grand visionaries such as Jefferson, Franklin, and Theodore Roosevelt conquered and settled a continent because of the seeming limitlessness of land and their belief that they had a divine mandate to do so.

In some ways, Mr. Kluger’s answers to the critical questions of expansion appear much like Frederick Jackson Turner’s. The frontier created Americans. On the other hand, he embraces the merits of the germ thesis as well. The ready temptation of land allowed these New World people to become what the Old World had prepared them to be: grasping conquerors. This sense of inevitability does not make for a novel or sophisticated interpretation, and the book does not pause over contingencies. But in fairness, Mr. Kluger does not take a triumphalist tone while covering an impressive amount of historical ground. While we don’t find new answers to critical questions in “Seizing Destiny,” the questions themselves are the right ones to ask.

Mr. Griffin is an associate professor of history at the University of Virginia. His book “American Leviathan” was recently published by Hill and Wang.