The legacies of historical figures stir debate far beyond their lifetimes. But there’s overwhelming agreement among Americans that today — Martin Luther King Jr. Day — honors a man who forced a long-overdue transformation for racial fairness in the United States.

As leader of the Civil Rights Movement, Dr. King held a mirror up to our nation.

He made us look at the bigotry-based injustice that still stained our land — particularly our South. Through nonviolent protest, relentless commitment and the ultimately overwhelming power of indisputable arguments, Dr. King won an epochal victory for human dignity.

Yet while he’s now widely regarded as a national icon, that wasn’t the case on April 12, 1963, when he was arrested in Birmingham, Ala., for defying a state judge’s order against “parading, demonstrating, boycotting, trespassing and picketing.”

At that point, many white Americans — again, especially in the South — did not approve of Dr. King and his methods.

And the Birmingham authorities thought they had dealt him a significant setback by locking him up.

Instead, they gave Dr. King an opportunity to collect his thoughts and write his remarkable “Letter from Birmingham Jail.”

While his “I Have a Dream” speech four months later in Washington stands as his signature message, the letter also endures as an epic proclamation of lofty resolve.

In it, Dr. King warned:

“Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly, affects all indirectly. Never again can we afford to live with the narrow, provincial ‘outside agitator’ idea. Anyone who lives inside the United States can never be considered an outsider anywhere within its bounds.”

And: “I have earnestly opposed violent tension, but there is a type of constructive, nonviolent tension which is necessary for growth. Just as Socrates felt that it was necessary to create a tension in the mind so that individuals could rise from the bondage of myths and half truths to the unfettered realm of creative analysis and objective appraisal, so must we see the need for nonviolent gadflies to create the kind of tension in society that will help men rise from the dark depths of prejudice and racism to the majestic heights of understanding and brotherhood.”

And: “One day the South will recognize its real heroes. They will be the James Merediths, with the noble sense of purpose that enables them to face jeering and hostile mobs, and with the agonizing loneliness that characterizes the life of the pioneer.”

Today, America — not just the South — recognizes the Rev. Martin Luther King Jr. as the real leader of that group of “real heroes” in the Civil Rights Movement.

Our nation also, thanks in large part to Dr. King’s efforts, recognizes the blatant injustice — and utter stupidity — of racism.

And while debate persists on how Dr. King’s message should be applied today, there’s no disputing his uplifting legacy — and his indispensable role in finally advancing our nation much closer to its founding ideals.