When the Civil War broke out in 1861, Washington, D.C. remained the capital of the fractured United States and also the military headquarters of the Union Army. Richmond, the newly minted capital of the Confederacy, was less than 100 miles away in neighboring Virginia.

Likening the Civil War to a chess game, the warring capitals of Washington, D.C. and Richmond represented the kings, says Kenneth Winkle, a professor of American history at the University of Nebraska Lincoln.

“The two capitals were so vital that the capture of either one would end the war,” says Winkle, author of Lincoln’s Citadel: The Civil War in Washington, DC.

At the outset of the Civil War, Washington, D.C. was extremely vulnerable to attack, defended by a solitary fort located 16 miles from the city center. But by the war’s end in 1865, Washington, D.C. was arguably the most heavily defended city on the planet, ringed by an impenetrable network of 68 earthen forts connected by miles of trenches, gun batteries and military roads.

The fall of Fort Sumter to the South Carolina militia on April 13, 1861, signaled the start of the Civil War, which was quickly followed by the secession of Virginia. Washington, D.C. now sat directly across the Potomac River from enemy territory.

“This is a very precarious position,” says Winkle, adding that Washington, D.C.’s other border was with Maryland, a slave state whose loyalty to the Union was shaky at best.

That’s why the Union Army’s first offensive action of the Civil War was to cross the Potomac into Virginia in the early morning of May 24, 1861 and capture high ground in Alexandria and Arlington, including the family estate of Confederate General Robert E. Lee (which eventually became Arlington National Cemetery).

The Union infantry quickly dug the first earthen forts, Fort Runyon and Fort Corcoran, to prevent the Confederates from installing cannons that could easily strike the capital.

But it was the Union Army’s shocking defeat in July 1861 at the first Battle of Bull Run—fought just 30 miles from Washington, D.C. in Manassas, Virginia—that convinced President Abraham Lincoln and Congress that the capital needed to be defended at all cost.

The formidable task of fortifying Washington, D.C. fell to Major General John Barnard, a respected Army engineer. Winkle says Barnard quickly recognized that the greatest challenge was Washington, D.C.’s sprawling layout, the result of architect Pierre L’Enfant’s ambitious grid design. The only effective way to defend all sides of the capital from attack, Barnard decided, was to establish a circle of fortifications surrounding the city.

Over the winter of 1861 and 1862, Barnard directed a team of Army engineers, soldiers, formerly enslaved people and prisoners of war to build the first 37 earthen forts that created a 35-mile defensive perimeter around the capital.

By the end of the war, the “Father of the Defenses of Washington”—as Barnard came to be known—constructed a total of 68 forts, each made with thick earthen walls that could absorb cannon balls and heavy artillery. Soldiers cleared the forest in front of each fort, dug a deep trench as a dry moat, and built up a barricade of sharpened tree trunks called an “abatis.”

In between the forts were 20 miles of earth-dug trenches known as rifle pits. Barnard also built 93 artillery batteries on prominent hilltops equipped with more than 800 cannons to cut down an invading force. Each fort was only manned by a handful of permanent soldiers, says Winkle, but the entire defensive ring was connected by more than 30 miles of freshly cleared military roads to speedily move thousands of Union troops to the site of an attack.

Barnard’s successor, Lt. Colonel Barton Alexander, even built a series of river obstructions to guard against a maritime invasion. Known as an “Alexandria Chain,” the apparatus consisted of floats holding up a 400-foot chain dragging 23 heavy anchors, but was never used in battle.

During the drawn-out conflict, the Confederate Army made several sorties in the direction of Washington, D.C.—Winkle says that both the Battles of Antietam and Gettysburg were primarily designed to threaten the Union capital—but the city only suffered one direct attack.

Lincoln wanted Washington, D.C. to be continuously defended by at least 30,000 regular infantry, but that wasn’t possible in the summer of 1864 when General Ulysses S. Grant desperately needed reinforcements in Virginia. By July, only 9,000 Union troops—mostly green new recruits and disabled reserves—were left to defend the capital and the Confederacy saw a golden opportunity.

Lieutenant General Jubal A. Early led 14,000 Confederate troops across the Potomac River into Maryland and then circled around to attack the Union capital from the north. On July 11, 1864, Early’s army arrived at Fort Stevens, where Lincoln himself stood with the shaky Union forces.

(Lincoln narrowly avoided being shot by a sniper’s rifle and had to be dragged off the parapet where his tall figure and top hat made him an easy target. Oliver Wendell Holmes, Jr., the future Supreme Court justice, fought at the Battle of Fort Stevens and claims to have yelled at Lincoln, “Get down, you damned fool!”)

Early and his men made a few probing runs at the fort, but quickly realized that even with their superior numbers, victory was impossible.

According to Early’s own words, the fortifications “were found to be exceedingly strong” and continued “as far as the eye could reach… of the same impregnable character.”

The dejected Confederate general concluded that “every appliance of science and unlimited means had been used to render the fortifications around Washington as strong as possible.”