When a group of mostly white college students traveled to the heart of the Jim Crow South to help African American citizens register to vote in June of 1964, they had no idea just how difficult the task would be, nor of the lasting impact it would have.

"I think it's really important that we understand the struggle that we had to go through to get people the right to vote and that probably that struggle isn't over," said Stanley Nelson, who wrote and directed the new PBS film "Freedom Summer."

Nelson's film, which premiered on PBS this week, recounts the stories of men and women - both black and white - who fought for right of African Americans to vote in Mississippi, a state where white supremacy and segregation were violently enforced.

"You'd be fired from your job if you even tried to go down and register to vote," Nelson told ABC News' Martha Raddatz in an interview for "This Week." "If you had a loan, any kind of loan, they would cut your loan… they would publish your name in the paper… Then there was something called the registrar who would then make you take a test and inevitably if you're African American, you fail."

For 10 weeks in the summer of 1964, volunteers from northern states traveled to Mississippi to join local community activists helping African American citizens with voter registration. They were strongly opposed by white supremacists, who resorted to intimidation, violence, and even murder to stop their efforts.

WATCH: A Clip from the PBS Documentary 'Freedom Summer'

"One of the terms that people always use for what was happening in Mississippi was terrorism. And you know, they were being terrorized," Nelson said.

Three young Freedom Summer activists - James Chaney, Andrew Goodman, and Michael Schwerner - disappeared on June 21, 1964, on the first day of the Freedom Summer movement. Their bodies were not discovered for six months, their murders later dramatized in the 1988 film "Mississippi Burning."

"The people who understood Mississippi knew that they were never going to be found alive," Nelson said. "It put this kind of shadow over the whole summer."

Still, this tragic event did not deter the other organizers. The Freedom Summer movement caught the nation's attention, and played a role in President Lyndon Johnson signing the Voting Rights Act the following year, banning voter discrimination.

The act remained intact until last year, when a sharply divided Supreme Court struck down a key part, citing racial progress in the last 50 years.

Nelson's film, filled with interviews with former Freedom Summer activists and their first-hand perspective of the events of 1964, is available in full on the PBS website.

ABC News' Brian Hartman contributed to this article.

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