In a country where movements of tired, poor and huddled masses are an intrinsic part of who we are, the unprecedented mass exodus of people from their homes in the Gulf Region – more than half a million refugees – could unleash changes for years to come.

“I think we’re looking at an event of enormous political and historical importance,” said Steven Hahn, a University of Pennsylvania history professor who chronicled other mass movements in his 2004 Pulitzer Prize-winning book “A Nation Under Our Feet: Black Political Struggles in the Rural South from Slavery to the Great Migration.”

“We’ve never faced this type of relocation because of a natural disaster. It’s likely to have an enormous impact on our entire country,” he said.

While many expect New Orleans to rebuild, it’s unlikely that everyone is eventually going to move back. Experts say there are lessons to be drawn from historic moves.

The size of the migration forced by Katrina is mindboggling. “There’s just never been anything like this before, with so many displaced people, and they’re going to be displaced for so long,” said Judith Owens-Manley, director of community research at Hamilton College’s Public Affairs Center in Clinton, N.Y.

For the Federal Emergency Management Agency, moving this many people has been overwhelming.

“This is the largest relocation of any sort of disaster response in our history,” said FEMA spokeswoman Natalie Rule. “We’re essentially relocating most of the city of New Orleans.”

The Barnett family occupies a makeshift shelter in Bakersfield, Ca., in this July 23, 1937, file photo. They were one of hundreds of American families who moved west to California to escape the drought, dust and disaster in their farmland in the Midwest Dust Bowl.

But war, economic disaster, religious and social persecution, and natural catastrophes have forced large populations to relocate many times in the nation’s history.

A movement that began around 1915, when more than a million blacks moved to Northern cities in about a decade in what was called the Great Migration, is one of them.

They were forced out by a sudden loss of jobs in the cotton fields which were suffering major boll weevil infestations, and enticed north with offers of jobs that were booming in the wake of World War I.

Just as that migration was coming to an end, the Dust Bowl exodus began. This, the largest migration in American history, saw 2.5 million people leave the Midwest and Great Plains, many heading west to California after drought, wind and economic depression destroyed their fields and their livelihood.