President Abraham Lincoln and General Ulysses S. Grant didn’t meet often in person. But their mutual respect and trust grew deep over the final year of the Civil War as they together steered America and its armies through the most convulsive period in the nation’s history.

In his memoirs, Grant confessed that he was “by no means a ‘Lincoln man’” in the years before the firing of the first shots of the Civil War at Fort Sumter, South Carolina. By the time General Grant accepted the surrender of Robert E. Lee at Appomattox, however, the cauldron of four years of war had forged a strong partnership between Grant and Lincoln—one that, for all intents and purposes, saved the Union.

“I think it was Grant’s aggressive, fighting spirit that endeared him to Lincoln,” says Ron Chernow, Pulitzer Prize-winning author of Grant. Not only was the general a self-starter, but he had a quiet self-confidence and a refreshing willingness to accept full responsibility for his battlefield defeats. “Too many of Lincoln’s generals were quick to scapegoat him for their failures,” says Chernow, “whereas Grant, as a matter of both pride and honesty, never blamed the president.”

Similar life stories bonded the men as well. Both overcame hardscrabble upbringings in the American heartland, married into slaveholding families and suffered periodic bouts of depression. With their modest Midwestern backgrounds came a shared democratic ethos: “Grant put on no airs with his men and treated officers and ordinary soldiers with similar courtesy,” Chernow says. “This appealed to Lincoln, who also showed a common touch with soldiers.”

With his prairie roots, Lincoln knew that the Civil War’s western theater and control of the Mississippi River would be vital to Union success, so Grant’s early victories in the region caught the president’s eye. While Lincoln seethed during 1862 at the plodding pace of General George McClellan and the Army of the Potomac, he admired Grant’s swift action in capturing Fort Donelson and Fort Henry in Tennessee.

When his troops were taken by surprise at the bloody Battle of Shiloh in April 1862 and floundered for months outside Vicksburg, Mississippi, Grant faced sharp charges of incompetence—and rumors of inebriation. A Republican senator denounced Grant to Lincoln as “bloodthirsty, reckless of human life and utterly unfit to lead troops.” The president stood by Grant and, by some accounts, even joked about sending a barrel of whatever Grant was drinking to the other generals. Nonetheless, Lincoln made sure to have his assistant secretary of war, Charles Dana, personally confirm his competence and sobriety.

In a possibly apocryphal tale, Republican politician and newspaper editor Alexander McClure reported that after he argued for Grant’s removal, Lincoln told him, “I can’t spare this man. He fights.” Real or not, the line has endured, largely because it so aptly captures why the president valued Grant. “Many Union generals temporized and put off battles until their troops were better trained and equipped,” says Chernow. “Grant recognized that such delays would benefit equally his Confederate opponents and preferred to strike quickly and capitalize on the element of surprise even when his troops weren’t perfectly ready.”

With his capture of Vicksburg on July 4, 1863, Grant seized the Confederacy’s last bastion on the Mississippi River. After Grant’s subsequent capture of Chattanooga and Knoxville, Lincoln initially resisted calls to give the general command of all Union forces because of the growing chatter that he coveted the White House for himself.

Once Grant made it known that he had no interest in political office, however, Lincoln elevated him in March 1864 to the position of lieutenant general, a rank previously held only by George Washington and Winfield Scott (who had received a temporary brevet promotion to that rank before the war). Not until three years into the Civil War did the two men meet for the first time when Grant came to Washington, D.C., to receive his new commission. Grant recalled that Lincoln told him at their first encounter that “all he wanted or had ever wanted was someone who would take the responsibility and act.”

Grant and Lincoln maintained frequent contact through the final year of the Civil War. If the taciturn general was ever critical of the commander-in-chief, he bit his tongue. “No general could want better backing, for the president was a man of great wisdom and moderation,” Grant recalled. When he decided to pursue Lee’s army after brutal losses in the Battle of the Wilderness, he dispatched a New York Tribune reporter with a message for Lincoln. “He told me I was to tell you, Mr. President, that there will be no turning back,” the correspondent reported. Thrilled to finally have a general he believed was taking the fight to the enemy, Lincoln kissed the reporter.

While Grant and Lincoln had a warm relationship, the same could not be said about their wives. When Mary Todd Lincoln flew into a jealous rage at the wife of General Edward Ord for the attention she bestowed upon the president in late March 1865, Julia Grant found herself on the receiving end of the First Lady’s acid tongue when she tried to intervene.

Several weeks later, on the morning of April 14, 1865, after Lee’s surrender, the president invited the Grants to accompany him and the First Lady to a performance of “Our American Cousin” at Ford’s Theatre. Lincoln wanted the public to see the victorious president and general together. Julia’s desire to avoid spending any more time with Mary Lincoln sealed Grant’s inclination to decline the invitation, and the general was not at the president’s side when he was assassinated.

Tears flowed down Grant’s cheek as he stood at attention next to the commander-in-chief’s coffin inside the White House and mourned the loss of a friend. “My personal relations with him were as close and intimate as the nature of our respective duties would permit,” Grant wrote. “To know him personally was to love and respect him for his great qualities of heart and head, and for his patience and patriotism.” Grant clearly had become, by all means, a “Lincoln man.”