On March 4, 1861, Abraham Lincoln climbed into an open carriage to travel to the United States Capitol to be sworn in as the country’s 16th president. There, in his inaugural address, he movingly called for unity in the deeply divided nation, famously appealing to “the better angels of our nature.”

It’s possible Lincoln might never have made it to Washington, D.C. at all. Openly despised by Southerners for his vocal opposition to slavery, he had been receiving daily death threats since his election.

One of those threats might well have been carried out—if not for the efforts of America’s first female private detective. Hired by famed Pinkerton Detective Agency, Kate Warne not only sussed out details of an assassination attempt against Lincoln, but she successfully fronted a scheme to foil the conspirators, who planned to ambush the president-elect during his train journey into Washington.

Before the creation of the Secret Service, presidents relied on the military to protect them. Lincoln loathed ostentation, though, and despite the volume of threats against his life, rejected any idea of a military escort on the lengthy and well-publicized train tour from his home in Springfield, Illinois to the nation’s capital.

One of Lincoln’s supporters, railroad executive Samuel Morse Felton, had grown alarmed both by rumors of conspiracies that involved Lincoln’s assassination and by the president-elect’s apparent unconcern. Looking for help, he turned to detective Allan Pinkerton. Not only had the Scottish-born sleuth established his business by providing security services to the railroad industry, but he had solid abolitionist credentials. Pinkerton had met Lincoln when both had worked on behalf of the Illinois Central Railroad, Lincoln offering legal advice and Pinkerton providing security.

Pinkerton, for his part, enlisted one of his most unlikely but most stalwart operatives to keep the president safe.

Only a few years earlier, in 1856, the widowed 23-year-old Warne, a long way from her birthplace in Erin, New York, had marched boldly into Pinkerton’s Chicago office in search of a job. And not just as a clerical assistant. She insisted she wanted to become an operative.

Pinkerton later chronicled his first encounter with Warne. “Female detectives were unheard of,” he said. Still, out of curiosity, he “asked her what she thought she could do.”

Warne reminded Pinkerton that there were places “to which it was impossible for male detectives to gain access,” but that, as a woman, she “could go and worm out secrets.” Struck by the young woman’s moxie, Pinkerton hired her, despite objections from his brother and business partner.

Within months, the young detective proved her value. Traveling to Montgomery, Alabama and cozying up to the wife of the prime suspect in the $50,000 robbery of the Adams Express Co., Kate Warne obtained a confession and recouped most of the loot.

“She succeeded far beyond my utmost expectations,” Pinkerton later raved in a memoir.

Warne’s time in Alabama would prove particularly useful when it came to keeping the president safe. She knew just how Southern belles talked and acted, so when Pinkerton sent her to Baltimore–the only slave-holding city that Lincoln planned to visit on his trip to Washington–she donned the black-and-white cockade of ribbons worn by secessionist sympathizers and posed as an Alabama society lady. Hobnobbing with the city’s elite, she helped to glean details of at least one major plot to kill the president.

How could Warne and Pinkerton best protect Lincoln? The conspirators planned to take advantage of the president’s well-publicized schedule, and the need for anyone heading for Washington to change trains in Baltimore. The would-be assassins plotted to take advantage of a staged scuffle at the crowded train station. Clearly, the Pinkerton’s team would have to smuggle Lincoln through Baltimore ahead of schedule and under cover of darkness. And they’d somehow have to disguise the all-too conspicuous president-elect.

Warne, undercover as “Mrs. Barley,” booked a sleeper compartment for herself in Philadelphia, telling a porter that her invalid brother would be joining her—and tipping him lavishly to keep the seats vacant and to leave the siblings alone during their long trip to the nation’s capital.

Pinkerton soon arrived at the station, escorting an almost-unrecognizable Lincoln. Gone was the stovepipe hat; in its place was a new, round beaver hat. Nothing could disguise Lincoln’s height, but as Warne solicitously assisted her “brother” to his seat, the president stooped and disguised his silhouette and prominent jaw by donning a shawl.

Lincoln, Warne later recalled, was surprised by the identity of his guardian, but remained gallant.

“I believe it has not hitherto been one of the prerequisites of the presidency to acquire in full bloom so charming and accomplished a female relation,” he told her.

Warne, meanwhile, was underwhelmed.

“Mr. Lincoln is very homely,” she wrote.

Throughout the long night that followed, Lincoln managed to sleep, but Warne sat bolt upright, alert to any threats. Instead of crossing Baltimore in broad daylight in an open carriage, the president’s railway car was pulled by a team of horses from one railway station to another and connected to the Washington-bound train in the middle of the night.

At dawn the next morning, Pinkerton sent a coded telegram to the railroad execs who had hired him, reading “Plums (Pinkerton) has delivered Nuts (Lincoln’s code name) safely.”

Pinkerton would live to bemoan the fact that he wasn’t responsible for Lincoln’s security during a visit by the president to Ford’s Theater in Washington four years later, soon after the South’s surrender ended the Civil War, a conflict that had begun shortly following Lincoln’s inauguration. Pinkerton also outlived Warne, his star agent, who died of pneumonia in 1868. Pinkerton buried her in his private family plot.