The rolling hills and forested ridges of Gettysburg, Pa., hold many stories about the clash of armies that occurred 150 years ago this week. But perhaps the most enduring is what would have happened if the South had won.

Would the direction of the war have shifted in favor of the Confederacy, or just prolonged its agony by a few more months? Would President Lincoln have been re-elected the following year, or would he have been turned out by a peace and accommodation movement led by Democrats?

Tens of thousands of visitors will be descending on Gettysburg National Park this week to commemorate the deadliest land battle in U.S. history, and remember the men who died there. At the same time, scholars of the Civil War continue to ponder the importance of this three-day fight that bloodied both sides, but led the Confederacy to retreat back to Virginia.

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One historian believes the battle between Confederate General Robert E. Lee and the Union's Army of the Potomac led by General George Meade truly was decisive

“If Lee had been victorious, the Army of the Potomac would have dissolved," said Alan Guelzo, history professor at Gettysburg College and author the new book “Gettysburg: The Last Invasion." “There were a number of soldiers who wrote before the battle about how the army had reeled from defeat to defeat, and if it happened one more time they would desert."

Guelzo firmly believes that the battle was decisive from a political standpoint as well. The Union army had lost at Chancellorsville weeks earlier, and Lincoln was facing trouble across the country. Not only did Lincoln have to manage the war, he also had to maintain support for his agenda of abolishing slavery. That wasn't as popular as we may believe today.

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After the Emancipation Proclamation was issued in 1862, the Republicans lost 36 members in the House of Representatives as well as the governorships of New York and New Jersey, Guelzo said. In the fall of 1863, the governorships of several states including Ohio and Pennsylvania were in danger of turning from Republican to Democrat. If that would have occurred, Guelzo believes they would have likely recalled their state militias from the Union army, leaving it weaker against the Confederates.

A loss at Gettysburg would have given the pro-peace Democrats the upper hand, he said.

Daniel Grill/Corbis

“The Democrats believed that Lincoln and the Republicans were a collection of radicals who were as much to blame as secessionists themselves," Guelzo said. “They believed the Republicans were radical abolitionists, and what we need to do is get the moderate people together."

Ten days after the battle of Gettysburg, which lasted from July 1 to July 3, 1863, the North began drafting young men, leading to the “draft riots" in New York, Boston and smaller cities like Toledo. Had Meade lost at Gettysburg and Lee began a military campaign in Pennsylvania and “you can imagine the political fallout in the coming weeks, and it's not going to be good for the Union."

As with all “what-if" games of history, not all historians agree with Guelzo's scenarios.

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“In the long term, the north had a winning strategy," said Elizabeth Varon, professor of history at the University of Virginia. “They had the numbers and the resources."

Varon notes that while Meade was ousted after Gettysburg, Gen. Ulysses S. Grant, was winning victories in the western part of the country, such as Vicksburg.

“The superior of leadership of Lincoln as president, and superior generalship and command harmony of Lincoln and his team in spring of 1864 was decisive," Varon said. “People raise the issue of Southern morale, and it would have been good (with a victory), but the once the Union had a team of generals who worked well with Lincoln and put advantages to bear, you were going to have a Union victory."

Meade's victory -- at a cost of 45,000 to 50,000 killed, wounded or missing on both sides -- sent Lee back home.

“The war in 1864 is being fought in Virginia," said Ian Isherwood, assistant director of the Civil War Institute in Gettysburg, Pa. “That means the south is defending its soil. That's a major difference."

As for the battle itself, many historians look back at the famous “Pickett's Charge" in which 12,500 units under Confederate General George Pickett attacked 3,500 Union forces on Cemetery Ridge. The Union held and the disastrous engagement on July 3 has since been referred to as the high water mark of the Confederacy. But Guelzo said the charge wasn't the only moment that could have tipped the battle, the war, and possibly American history, the other way.

“There were a dozen times where the difference of 15 minutes, or a junior officer's decision might have meant difference between victory and defeat," said Guelzo, who lives on part of the battlefield. “It's not a static affair. It's playing multiple rounds of Russian roulette."