Nearly every facet of Abraham Lincoln’s life has been explored in the 148 years since his assassination. What could a new Lincoln biography offer?

For author David Von Drehle, the answer lies in tracing Lincoln’s transformation from inexperienced new president of a crumbling country to steady-willed leader who saved the Union in its most vulnerable hour. Von Drehle, a journalist and historian with impeccable writing chops, limited himself to a single year of Lincoln’s presidency – 1862, which he calls “America’s most perilous year.”

He set out to explore how Lincoln even survived the year, and ended it stronger than before. Von Drehle immerses the reader in Lincoln’s daily life in 1862. We are at Lincoln’s side as he struggles to persuade, cajole and finally order his generals to use their superior numbers to bring the war to the South. We watch as the heartbroken Lincolns suffer the death of son Willie. We see, too, his sometimes Machiavellian efforts to keep his fragile coalition of a cabinet united around his primary goal of preserving the Union.

Most important, we observe the evolution of Lincoln’s thinking on the crucial issues of the day.

In one particularly effective scene, Von Drehle finds meaning in the incomplete thoughts Lincoln left on a scrap of paper one day in September.

“The will of God prevails,” Lincoln wrote. And despite the mounting human carnage, God “wills that (the war) should not end yet.”

But why? Lincoln’s note ends without an answer. Von Drehle argues that in that moment, Lincoln began to believe that ending slavery was the real reason for the war. The author fast-forwards to Lincoln’s second inaugural address in 1865, when he publicly postulated that God’s purpose in allowing the hideously bloody war to continue must have been to abolish the offense of slavery.

Von Drehle surmises that this led Lincoln to a decision. If the Union army could thwart a rebel invasion into Maryland, then the president would make slavery a centerpiece of the war by issuing his Emancipation Proclamation, freeing slaves in rebel territory and setting the stage for the final eradication of slavery in the 13th Amendment.

Aided by the chance discovery of secret Confederate battle orders, the Union did repulse the rebel invasion. But once again, Confederate Gen. Robert E. Lee escaped from slow-footed Union Gen. George McClellan. Von Drehle explores the exquisite pressures Lincoln faced as he dealt with McClellan, a proud and mercurial young general.

McClellan was fervently supported by pro-Union Democrats across the North. He also was lambasted by Northern radicals who felt that “Little Mac” was sabotaging Union efforts and taking it easy on Lee, perhaps to aid his own political future.

Amid such furor, Lincoln navigated a steady course, visiting and wiring McClellan with encouragement that turned to impatient commands as the year went by. Von Drehle uses McClellan’s letters to his wife to show the general’s utter contempt for the commander in chief, as the general referred to Lincoln as a “gorilla” and worse.

Finally, after his party survived midterm elections with little damage, Lincoln felt strong enough to get rid of McClellan. (To his credit, the general resisted pleas to attempt some sort of coup, preferring to wait until the 1864 election to challenge Lincoln at the polls.)

No matter how long you look at Lincoln, Von Drehle concedes, you never really get to his core. But Von Drehle puts the reader as close as anyone has to Lincoln’s mind in a crucial year. He deftly explores Lincoln the leader as well as the man, tracing the evolution of his thinking as the full horror of the war was only beginning to make itself known.