Ask noted filmmaker and documentarian Ken Burns why the American Civil War still matters nearly 150 years after it started and he'll tell you it's the single event in U.S. history which matters most -- and in some ways is still playing out today.

"More traumatic than 9/11 by far," Burns said of the Civil War at the opening of a groundbreaking exhibit commemorating its sesquicentennial at the National Archives. "This is the searing event in our nation. Nothing comes close."

More than 2 percent of America's population, or 620,000 people, died in the four-year conflict that began in April 1861 over social, economic and political divisions centered on slavery. By comparison, 418,500 Americans, or less than one half of 1 percent of the population, died in World War II.

The Civil War led to the abolition of slavery, charted a course for the modern civil rights movement and defined the relationship between citizens, the states and their federal government, Burns said in an interview with ABC News.

"We found out how to be one thing -- a nation," he said. "We found out how to say the United States 'is' rather than the United States 'are.'"

Burns, the country's de facto "Civil War buff-in-chief," has been credited with educating generations of Americans through his Emmy Award winning PBS documentary "The Civil War," which has been shown in classrooms and community centers across the country for nearly 20 years.

He was also instrumental in developing the Archives' most extensive exhibit of war records to date and the first in half a century to focus exclusively on the Civil War. It debuts in Washington, D.C., on Friday.

Walking past colorful thematic displays that explore questions of how armies were raised, leaders were chosen, and resources were mustered, Burns reflected on the poignancy of the Civil War's legacy a time when today's America is gripped by partisan rancor, vitriolic ideological debate, and threats against government and some of its leaders.

"I don't think we're coming apart now," said Burns of the present political divide. "But it's a manifestation of some of the elements that came together in the Civil War."

Burns Sees Legacy of Civil War in Current U.S. Partisanship

Scholars say issues of race, sectional economics and federalism, the relationship between states and the federal government -- all central to the debates of the 1860s -- continue to reverberate 150 years later, evident in the fight over Arizona's immigration law and Tea Party protests against Democrats' health care reform.

Even history of the war itself has become part of the contemporary ideological fray, most recently when Virginia Gov. Robert McDonnell declared April "Confederate History Month" in his state.

McDonnell's proclamation caused a firestorm for failing to acknowledge the role of slavery in the war or that it was championed by the Confederacy. The governor later apologized.

"As long as we exclude huge portions of our past and use the past to serve as a political vehicle now, we do a disservice to everyone," said Burns. "The Civil War involved 4 million human beings that were owned by other human beings in a country that had declared to the world all men are created equal. That's why the Civil War happened."

Burns also pointed to festering debate over Confederate symbols in state flags as a sign that the country's racial and political wounds have yet to fully heal.

"If that's [Confederate symbol] been there from the very beginning, since before 1861, then it should be part of the heritage," Burns said. "But if it was something that was added, as so many of the flags were in 1954 in response to [desegregation of schools], then I think we know what's going on. When we hear people using code words about some aspects of the past, we have to be very careful."

National Archives curators say the new exhibit, "Discovering the Civil War," peels back many of the "accumulated myths and punditry" to allow visitors to draw their own contemporary parallels.

The 6,000 square-foot display debuts this week at the Archives' home on the National Mall. It will eventually travel the country with stops in Dearborn, Mich., Houston, Tex., Omaha, Neb., and other destinations beginning next spring.