The drama of the American Revolution, as it plays out in most of our memories and imaginations, is set mainly on the Atlantic seaboard. From Lexington down to Yorktown, the major defeats and victories are associated with the densely populated eastern fringe of the 13 colonies. Likewise, in the early Republic, it was big cities that formed the stage for history – the first capital in New York, the constitutional debates in Philadelphia.

Yet during all that time, one of the most important arenas where the future of the country was being decided lay far to the West, in the thinly settled reaches of New York State. From the Mohawk River valley to Lake Erie, and from the Canadian border to Pennsylvania, lay a nation that never appeared on any map: what Alan Taylor, the renowned historian of colonial America, calls “Iroquoia.” This was the territory of the Iroquois Six Nations, the powerful Native American confederacy that lay directly athwart the path of New York’s westward expansion, forming both a bridge and barrier between the United States and British Canada. Before the Revolution, the Six Nations were a major player in the geopolitics of North America. But by the time of the War of 1812, they had been largely dispossessed, their land taken by speculators and their political power shattered.

How this happened, and what it meant for the future of the republic, is the subject of Mr. Taylor’s dense and learned new book, “The Divided Ground” (Alfred A. Knopf, 542 pages, $35). In Mr. Taylor’s best-known work, the Pulitzer Prize-winning “William Cooper’s Town,” he approached the history of this time and place through the microcosm of a single upstate New York village. “The Divided Ground,” by contrast, is large-scale history, charting the whole saga of diplomatic, economic, and cultural exchange among Americans, Canadians, and Iroquois over several decades.

Mr. Taylor does try to anchor this complex story in a few vivid biographical portraits, focusing especially on Joseph Brant, a bicultural Iroquois power broker, and Samuel Kirkland, a Protestant missionary. But while portraits of these two men adorn the book’s cover, “The Divided Ground” treats them only incidentally, as they play greater or lesser roles in the immensely complicated story Mr. Taylor has to tell. There is no disguising the fact that “The Divided Ground” is often slow going, as Mr. Taylor recounts each phase of the American-Iroquois conflict in minute detail. But readers who persevere to the end will be rewarded with a new understanding of American history, and of the sordid schemes hidden under the grand slogan of Manifest Destiny.

For most of the colonial period, the Six Nations engaged in a delicate diplomatic game, playing off the British against the French in order to maintain their independence and their supply of manufactured goods. In colonial New York, the remarkable Sir William Johnson operated as a virtual one-man government, using his unique knowledge of Iroquois customs to keep the peace between settlers and natives. Johnson was hardly a disinterested, benevolent figure – he used his position to become one of the biggest landowners in the colony – but he won great respect from the Iroquois, and his shadow looms over the events in “The Divided Ground.”

By the time Mr. Taylor’s story begins, this delicate balance had broken down forever. William Johnson was gone – he died in 1774, just before the Revolution he would certainly have opposed – and the men who took his place, including his son and son-in-law, had little of his talent. More important still, the French had been driven out of Canada in the Seven Years’ War, leaving the Iroquois without their traditional source of leverage. Under a new generation of leaders, including Brant – whose sister Molly had been Johnson’s common-law wife – the Iroquois struggled to find a way to hold off the tide of land-hungry American settlers.

But as Mr. Taylor shows in great detail, the deck was stacked against the Iroquois. First was the basic fact of population. Because of their low-impact use of the land – small farming plots, complemented by extensive hunting and gathering – the Iroquois needed a vast territory to sustain a relatively small number of people. Colonial agriculture could support much greater numbers, giving the Americans both a demographic advantage and a powerful incentive to take over Indian lands. As the Revolutionary War general Benjamin Lincoln argued, “I am confident that sooner or later … no men will be suffered to live by hunting on lands capable of improvement, and which would support more people under a state of cultivation,” especially since “the original plan of the benevolent Deity” was “to people fully the earth.”

If the Americans had a greater appetite for land, their second advantage came from their control of the legal system of land ownership. Under Governor George Clinton, New York State held that it alone had the right to purchase land from the Iroquois, imperiously upholding this monopoly against challenges from the federal government, other states, and private individuals. As a result, the Iroquois could not bargain for the best terms when they sold land, and couldn’t lease land at all, even though its value was bound to go up over time. Again and again, the state extorted huge parcels of land from the tribes at rock-bottom prices, then resold it to speculators at a huge profit. This tactic helped strengthen the young government’s authority and allowed it to pay back its huge war debts; but New York’s gain was the Six Nations’ loss. Well before the War of 1812, their land had been sold off piece by piece, until nothing was left but a few reservations surrounded by American settlers. The birth of America meant the death of Iroquoia, as the Oneida chief Captain John saw as early as 1793:

Now it appears you were very fortunate, you conquered your enemies with our assistance and obtained your Independence and are now rich and hap py, as you yourselves confess. … We have but a little land left, and if this is gone we shall have nothing, we shall weigh nothing in the scale of nations, and you will pay us no kind of respect. It does not appear that you care anything for us but to get our land.

As “The Divided Ground” conclusively shows, it was the simple truth.