Mine eyes have seen the glory...
There's an episode of The Johnny Cash Show from 1969 where Cash makes a little speech with what seemed at first to be a pretty big error.
"Here's a song that was reportedly sung by both sides in the Civil War,"
Cash said, guitar in hand, to kick off a performance of Battle Hymn of the Republic. The real history on that point is clear. The song was written as a pro-Union, anti-slavery anthem. But then Cash goes on to say,
"...which proves to me that a song can belong to all of us." 
And about that, he's correct.
Glory, glory, hallelujah!
Battle Hymn of the Republic went through a number of versions in the years immediately prior to the Civil War. Its tune and its early lyrics were written by William Steffe, ironically a southerner, in 1856. Its first verse and refrain were:
Say brothers, will you meet us?
Say brothers, will you meet us?
Say brothers, will you meet us?
On Canaan's happy shore?
Glory, glory hallelujah!
Glory, glory hallelujah!
Glory, glory hallelujah!
For ever, evermore!
The song first gained popularity around Charleston, South Carolina, where it was sung as a Methodist Camp Meeting song, particularly in churches belonging to free Blacks. By contrast, it was also used early on as a marching song on army posts. 
The song gathered new verses following the insurrection at Harper's Ferry, led by John Brown and carried out by a cadre of nineteen men on October 16, 1859. Brown's actions, trial, and subsequent execution (he was hanged for leading an attempted slave insurrection) made him a martyr to Abolitionists and African-Americans and prompted some people to add the following lines to Steffe's by then popular song.
John Brown's body lies a-mouldering in the grave.
John Brown's body lies a-mouldering in the grave,
John Brown's body lies a-mouldering in the grave,
His soul is marching on!
By the time of the Civil War, John Brown's Body became super popular among the Union soldiers for a few reasons. For one, the simplicity of the lyrics and melody made it easy to sing, and to remember. More importantly, it glorified the righteous fight against slavery. African-American units picked up the melody and added their own spin:
We're done with hoeing cotton,
we're done with hoeing corn,
we're colored Yankee soldiers
just as sure as you were born.
The Twelfth Massachusetts Regiment, in particular, has been credited with spreading the song's fame on their march to the South.
But it was when a well-to-do, highly educated poet from New York named Julia Ward Howe (1819-1910), the wife of a prominent Boston abolitionist, came to Washington, D.C. in 1861 that the tune properly came to be called Battle Hymn of the Republic. Howe and her minister, Rev. James Freeman Clarke, visited a Union Army camp in Virginia where she heard soldiers singing a tribute to the abolitionist John Brown. The Confederates attacked - but the Union soldiers defended and impressed Howe.
Her minister, aware that Howe occasionally wrote poetry, suggested that she craft new verses more appropriate to the Civil War effort, to be set to the same rousing tune - "rewrite it and elevate it with more uplifting words. Make that song richer for a kind of educated audience."
As Howe later explained it, the verses came to her in a single night:
"I went to bed and slept as usual, but awoke the next morning in the gray of the early dawn, and to my astonishment found that the wished-for lines were arranging themselves in my brain. I lay quite still until the last verse had completed itself in my thoughts, then hastily arose, saying to myself, I shall lose this if I don't write it down immediately. I searched for an old sheet of paper and an old stub of a pen which I had had the night before and began to scrawl the lines almost without looking, as I learned to do by often scratching down verses in the darkened room when my little children were sleeping. Having completed this, I lay down again and fell asleep, but not before feeling that something of importance had happened to me."
Battle Hymn of the Republic by Mrs. Julia Ward Howe. Published by the Supervisory Committee for Recruiting Colored Regiments. [n.d.] Music Division
Soon afterward, Howe submitted the poem to The Atlantic Monthly, which accepted it and paid her a fee of four dollars. After the verses appeared on the first page of the February 1862 issue, they quickly caught on as the rallying anthem of the Union troops and were sung frequently throughout the rest of the Civil War.
Julia Ward Howe was quite socially active in circles that included authors such as Charles Margaret Fuller, Charles Sumner, Ralph Waldo Emerson, and Charles Dickens. A lyrical poet and prolific author in her own right, she is best remembered for her new words to the Battle Hymn of the Republic. She was active in prison reform, the fight to end slavery, and the fight to win equal rights for women. Her efforts for social justice continued after the war. In 1870, she campaigned for a Mother's Day for Peace, a precursor to Mother's Day. She also pushed for women's suffrage - the right for women to vote.
Howe's words later inspired American soldiers during World War II and civil rights activists during the sixties. The day before he was killed in 1968, Martin Luther King Jr. made his famous "I've Been to the Mountaintop" speech, which he ended by quoting the song's first line:
Mine eyes have seen the glory of the coming of the Lord.
His home church, Atlanta's Ebenezer Baptist, took up the song after his death as an anthem to him and the civil rights movement.
His truth is marching on!
"How people come to patriotism is kind of how they come into Battle Hymn of the Republic," says professor Brigitta Johnson, an ethnomusicologist at the University of South Carolina who teaches in the schools of Music and African-American studies. In other words, Johnson says, "This anthem is all about what you bring to it." And in fact, that flexibility is part of its design. "It's really about supporting whatever your perspective is - about freedom or liberation, and having God as the person who's ordaining what we're doing. And 'glory, hallelujah' about that."
Mrs. Howe's hymn has been acclaimed through the years as one of our finest patriotic songs. At one time, it was sung as a solo at a large rally attended by President Abraham Lincoln. After the audience had responded with loud applause, President Lincoln, with tears in his eyes, cried out, "Sing it again!" It was sung again. And after more than 160 years, Americans still join in proclaiming,
Glory! Hallelujah! His truth is marching on!
As Johnny Cash said in 1969, "Battle Hymn of the Republic is an anthem that belongs to everybody. But what really matters is what they're singing it for."
Lewis, Jone Johnson. "Battle Hymn of the Republic: First Published Version." ThoughtCo, Jul. 31, 2021, thoughtco.com/battle-hymn-of-the-republic-words-3528494.