During the Revolutionary War, a good map could mean the difference between victory and defeat, especially for British troops facing guerrilla tactics for the first time. Few maps had been made of the interior of what would become the United States, and commanders often had to rely on maps drawn by soldiers in the midst of the fighting. (Read 4th of July: Nine Myths Debunked.)

The new book Revolution: Mapping the Road to American Independence collects many such battle-tested maps, which give fresh perspectives on the period’s key conflicts. Speaking from Washington, D.C, authors Richard Brown and Paul Cohen explain what Harry Potter and maps have in common; why maps incite wars; and how some cartographers were badly wounded as they drew their maps. (Read about battlefield artists during the Civil War.)

View Images Courtesy of W. W. Norton and Company

You write that “cartography always benefits from war … sometimes maps are even the cause of warfare.” Unpack that idea for us.

RB: At the beginning of the conflict between the French and English, during the early stages of the French and Indian War, both countries were making claims on the same land. With two major powers competing for land that was not very well defined on the maps, they were both exaggerating their own possessions. The war that resulted was a fight over the claims being made on these maps.

Give an example of how geography affected a battle’s outcome.

PC: The first major battle of the French and Indian War was when General Braddock came over from England with 2,000 English and Irish troops to take Fort Duquesne from the French in 1755. He had been told it was about a 45-mile trip from Alexandria, Virginia, to Fort Duquesne, which is modern day Pittsburgh, and that they could get there by going up the falls of the Potomac and sailing right on up.

It turned out to be over 240 miles. Braddock had to cross the Alleghenies, using block and tackle from the naval vessels. He insisted on taking as many 12-pound cannons as he could, which required teams of 30 horses to drag them. And he had to chop a path through the forests along the entire distance with 300 axmen.

View Images Famous as the location for Hogwarts in the first two Harry Potter movies, Alnwick Castle also houses a major collection of Revolutionary War maps. Photograph by Design Pics Inc, National Geographic

The troops had endured a long sea journey to get to Virginia and had marched for three months. But in a matter of hours, they were ambushed and destroyed by an inferior-sized French and Indian force because they didn’t understand the nature of bush warfare.

That was the first indication that the tactics and maps that had been developed in London were in no way applicable to the topographical situation in America.

Many of these maps were drawn by eyewitnesses to the battles. How do they communicate the immediacy of combat?

RB: The best visuals we have of North America, including Canada, during that period come mostly from British military officers. The maps we were especially interested in were those created closest to the action: manuscript maps done by an engineer or artillery officer on the field of battle. The conditions they created these maps under were horrific. They were not in some administrative group in the background. They were in the front lines and one of their jobs, in addition to manning the artillery or building fortifications, was to map the entire area.

View Images The lack of decent maps of the American interior meant the British were ill-prepared for bush warfare, as shown in this painting by Edwin Willard Deming of a Native American unit ambushing General Braddock’s army near Fort Duquesne, Pennsylvania. Photograph by MPI, Getty Images

One map, by Lieutenant Thomas Hyde Page, shows Bunker Hill. Page was at that battle and was so seriously wounded that he received a pension for the rest of his life. His map, which has been in the Library of Congress since the 19th Century, shows all the action: the fire, where the bullets are being shot, the range of the bullets, all done by someone who was actually wounded in battle.

You write that “the Golden Age of British cartography had its beginning at Quebec.” Why Quebec?

PC: In 1759, during the French and Indian War, a British officer who had been held prisoner there designed a map of the fortifications of Quebec. This was incredibly powerful: a map that could be used in battle. Nobody in England had any maps of Quebec, so General Wolfe, the hero of the Battle of Quebec, used that map.

View Images The charts of the St. Lawrence River made under cover of darkness by Captain James Cook enabled British General James Wolfe, above, to capture the strategic city of Quebec during the French and Indian War (1754-63). Photograh by Time Life Pictures, Getty Images

The second key thing was Captain James Cook’s sea chart. Cook went on to make his name as a global navigator, but he was also at Quebec. Wolf’s troops had to move from Louisburg to Quebec, about 1,000 miles through the Cabot Straits, all the way down the St. Lawrence. It was deemed to be unnavigable, but in one of his first actions Captain Cook went out at night and mapped the river channels and shoals. This allowed the British fleet to pass through an area that the French thought was impassable. So by the end of the Battle of Quebec you have these major mapmakers, who will become important throughout the Revolution.

Many of the maps you have gathered together have never been reproduced before.

RB: One thing we discovered is that some of the best collections of Revolutionary War maps have been among the least used. Historians tended to use the same maps over and over again to illustrate their narratives. What we did is to take the opposite view. We wanted the maps to tell the story, so we picked maps that we thought would tell the story of the battles best. Some of the most important maps from Lafayette’s battles or of commanders like General Erskine, whose map is on the cover of our book, had never been seen before. But it’s the best map of New York done at the time of the war.

Alnwick Castle in England is famous as the setting for Hogwarts in the first two Harry Potter movies. But that’s not why you went there, is it?

RB: [Laughs] Lord Hugh Percy, whose castle this was, had been at some of the most important battles of the early Revolution—Lenox, Concord, New York, Bunker Hill—so he was right in the thick of things at the beginning of the Revolution. We learned that Lord Percy’s wonderful map collection was housed at Alnwick Castle and so we knew we had to go there. It’s near the Scottish border, a huge castle, and the archivists were very welcoming, maybe because they don’t get many visitors up there in the far north of England. Many of the maps had never been reproduced and when they had, they had been done in small, uncolored versions. We have illustrated them in full color to the best of their advantage.

View Images John Jay used a copy of this 1755 map of North America by British cartographer John Mitchell to negotiate the Treaty of Paris that ended the Revolutionary War. Jay’s copy still “shows the lines of the new borders of this new country,” says author Paul Cohen. Illustration by Buyenlarge, Getty Images

Why is the Mitchell map considered “the most important map in American history”?

PC: Probably because John Mitchell, who was an Englishman living in America, incorporated every bit of geographical knowledge available to him. It was published in 1755, which is called “The Year of the Great Maps,” because there were three or four maps produced that year which had enduring qualities. The map was so complete and the knowledge so good that it remained in print as the standard map of America throughout the rest of the 18th century and was used during the 19th century and even in the 20th century to resolve border disputes.

It was one of the causes of the French and Indian War because it showed claims of the British possessions, which was one of its purposes in the first place. It was also used for the treaty at the end of the Revolutionary War. King George had a copy as did John Jay, whose copy is in the New York Historical Society and shows the lines of the new borders for this new country.

So it was the cause of warfare and was also used to make peace.

This interview was edited for length and clarity.