The stakes in this year's presidential campaign are high. But that's nothing new. There have been many other pivotal presidential elections in our history, some that set an entirely new course for the United States and a few that were crucial to the very survival of the republic. To put the current campaign in perspective, U.S. News White House correspondent Kenneth T. Walsh, author of four books on the presidency, examines the 10 most consequential elections in American history—the races that produced the biggest change and had the most lasting impact. An installment of this 10-part series will run on the U.S. News website each Wednesday through September. This is the second in the series.

On April 12, 1861, about five weeks after Abraham Lincoln's inauguration for his first term, Southern forces began bombarding Fort Sumter in Charleston harbor, starting the Civil War. On April 13, the Union forces surrendered, prompting jubilation in the new Confederacy and anger and disappointment in the North.

The election of Lincoln, an antislavery moderate, had been the last straw for pro-slavery leaders. His victory ended any hope they had of compromise because they were convinced that, with Lincoln in command, the North would trample the rights of the Southern states and move to end slavery. They began to secede from the Union and form a Confederacy of their own.

Many in the North thought the war would end quickly, but they didn't properly assess the military strength and the will of their adversaries, the quality of the Confederate forces' leadership, and the difficulty of invading and pacifying the Southern states. As a result, the war went very badly for the Union at first, and Lincoln's popularity in the North plummeted. He was derided as a despot, a dictator, an incompetent, and worse. At the same time, he was blamed for the many failures on the battlefield and for the horrendous casualties, posted day after day in town after town across the land. He changed generals when they lost big battles or when they didn't follow up on their limited successes, but for the early years the conflict seemed hopeless.

Meanwhile, Lincoln worked tirelessly to keep the Republican Party behind him and to minimize antiwar sentiment in the North. Many historians say he handled a very bad situation as well as anyone could have. "If there is a common denominator in presidential assessments, it is a bias toward activism, unless the activism is viewed as misplaced, as in the instances of Lyndon Johnson and Vietnam, and Nixon and Watergate," says Princeton's Fred Greenstein. Lincoln made his share of mistakes, such as choosing a succession of inept commanders, but he acted decisively and wisely when it counted most.

One of his biggest decisions—now considered a huge advance in American justice—was issuing the Emancipation Proclamation on Sept. 22, 1862, which ordered freedom for slaves in the rebellious states. This was considered a half measure by some abolitionists because it failed to free slaves in all the states. But it set the Union on the path to ending slavery eventually. It also had the effect of shoring up support for the Union in Europe, especially in the United Kingdom, which had been flirting with the Confederacy. The proclamation cost the South some important potential allies because many European leaders didn't want to back the slaveholding Confederacy and abandon the antislavery North.

Then, in 1863, the military tide turned—and the North's military success became the most important political development of the 1864 presidential campaign. Union forces won at Gettysburg and Vicksburg, and Lincoln's stock began to rise.

In 1864, there were more Union successes, including those at Atlanta and Mobile Bay, and Lincoln got much of the credit. With the war now seemingly on a positive track, he easily won a second term. He received 2.3 million votes to former Union Gen. George McClellan's 1.8 million. Even though McClellan had been Lincoln's senior commander, the former general sought an early end to the war, which was a popular position for most of the campaign. But the North's newfound military success undercut his arguments that it was time to sue for peace. Perhaps most gratifying to Lincoln, the soldiers doing the fighting gave Lincoln a huge margin, 116,887 votes to McClellan's 33,748, even though they knew that re-electing Lincoln would mean continuation of the conflict and the likelihood that many of them would be killed or wounded. But they also knew that re-electing Lincoln would virtually guarantee victory, complete with the end of slavery and the preservation of the Union, and these were their top priorities.

On March 4, 1865—in his second inaugural address—Lincoln gave one of the most eloquent and stirring speeches in history. "With malice toward none, with charity for all, with firmness in the right as God gives us to see the right," he said, "let us strive on to finish the work we are in, to bind up the nation's wounds, to care for him who shall have borne the battle and for his widow and his orphan, to do all which may achieve and cherish a just and lasting peace among ourselves and with all nations."

The following month, five days after Gen. Robert E. Lee surrendered his Confederate forces at Appomattox, Lincoln was shot by an assassin. He died the next morning, on April 15, 1865.

In the end, Lincoln's profound legacy was created and propelled by two elections—the one in 1860, which triggered the war, and the election of 1864, which enabled Lincoln to win it. Historian Henry Adams once wrote that a president "resembles the commander of a ship at sea. He must have a helm to grasp, a course to steer, a port to seek." Lincoln understood this to his core. Added historian Arthur Schlesinger Jr.: "The Constitution offers every president a helm, but the course and the port constitute the first requirement for presidential greatness. Great presidents possess, or are possessed by, a vision of an ideal America. Their passion is to make sure the ship of state sails on the right course." Defining that vision and setting that course are what Lincoln's presidency was all about.