80% of the South owned no slaves, yet they fought for slavery. Why?

The Civil War was fought by poor whites to avoid integration with blacks.
The question was not resolved by the Civil War which only eliminated formal slavery.

After the Civil War there was a short 12 year period of integration imposed by the Federal troops stationed in the South. As soon as the Federal troops left, racial segregation was started in the South by 1877, and continued to 1964. The Civil War did not resolve the issue and one can argue it has never been solved and a dormant Civil War is still being fought.

In the 1800's, It was even conceivable to consider poor whites lower than slaves as the following plantation song suggests:
Nigger pick de cotton, nigger tote de load,
Nigger build de levee foh de ribber to smash,
Nigger nebber walk up the handsome road,
But I radder be a nigger dan po' white trash!

(as quoted in Gwen Bristow's Plantation Trilogy).

Author Gwen Bristow writes, that according to the 1850 Census 80% of the population of Southern states were non-slaveholding, poor whites (footnote 1). Poor whites were poor because they had to compete for jobs with slaves who were paid nothing. They did not like slavery but they were also put off by the culture of African slaves and wanted to stay segregated from them.

Why would these southern poor whites, who had no interest in defending slavery, risk their lives in a war to uphold slavery, was a mystery to me, until I found the answer in a 2011 article: Why Non-Slaveholding Southerners Fought, by historian Gordon Rhea (https://www.battlefields.org/learn/articles/why-non-slaveholding-southerners-fought). According to Rhea the two main reasons why poor white, non-slaveholders fought the Civil War were:

1.The Civil War was fought for Segregation as much as for Slavery
Southern Politicians scared the 80% of the population that were poor white, non-slaveholders, with the idea that Northerners wanted to impose freed blacks on them as equals, and even as part of their government. Poor white, non-slaveholders wanted no part of this scenario and were willing to fight a war to avoid it.

2.The Civil War was preached as a Religious War
The fact that the Bible in some passages accepts the status of slaves as normal, was interpreted by Southern Presbyterian, Methodist and Baptist preachers as justifying slavery in the eyes of God. To be against slavery was considered equivalent to doubting the word of God in the Bible. According to them the Northern Abolition movement was an attack on Christianity. According to historian Rhea there are many surviving sermons from Southern Churches expressing this point of view. Those three denominations split into Northern and Southern Denominations because of the disagreement about slavery being sanctioned by God.

These two reasons together were enough to convince poor white, non-slaveholding Southerners to risk their lives in the Civil War.

Confusing Historical Terminology
It is confusing that the Historical periods of Civil Rights' fights ended up being designated by the name of the type of work being done after the Civil War, first to "reconstruct" the destruction of the South, and afterwords to "Redeem-Reverse" the perceived damage done to Southerners by the freeing of slaves and their election to offices during the Reconstruction period. The terms "Reconstruction" and "Redemption" are very confusing when they are used to label the status of blacks after the Civil War.

Segregationist phases:

1.Civil War (1861-1865)

2.Reconstruction (1865-1877) Emancipation from Slavery - The period of the reconstruction-rebuilding of the destroyed South coincided with the implementation of the Emancipation from Slavery by the Federal Government. Many blacks became part of local governments supported by the votes of abolitionists and ex-slaves . This was the situation the poor white population had been so afraid of, and after 1878 they managed to reverse it in what was called the "Redemption" movement.

3.Redemption (1878-1964) Reversal of most rights of Emancipation from Slavery - For about 86 years Southern State Governments’ implemented Jim Crow laws that replaced Slavery by Segregation under the doctrine or “equal but separate.” This reversal was the result of the weakness of the Northern Republican party which in order to hold on to power needed the vote of Southern white voters. In exchange for the Southern vote the Republicans agreed to allow the south to rule themselves through their state governments. It was called Redemption because after the war for a decade blacks were elected to offices and whites resented their power loss until they were “redeemed” by the laws of this State Rights’ political movement that instituted legal segregation within most Southern States. This reversed the Emancipation from slavery in everything but the label "slavery"). This legal segregation was only removed from law 86 years later with the passage of the 1964 Civil Rights Act.

4.Civil Rights Struggle (1960-1964) Culminating in the 1964 Civil Rights Act.

5.Continuing Segregation (1965-present) The old racist segregation remained culturally in place, and continues to surface intermittently in populist movements, in black vote-suppression, and in similar States Rights’ political movements, and even recently in toleration of white supremacist groups.

6.White Fragility a best-seller by Robin Diangelo (2018) Diangelo claims that white's outraged reaction at being unfairly accused of racially charged statements about blacks (which she calls "white fragility"), causes liberal whites to avoid examining the history of injustices against blacks and unwittingly gives cover for the injustices to continue. (2018 unemployment blacks 6.5% vs whites 3.1%). While the sociological outcome of prolonging racism seems correctly observed, the intentional complicity of white liberals in racism, which Diangelo claims, seems unjustified. Most white liberals are unaware of the history of injustice against blacks, not because of their complicity in white racism, but rather because of the inadequate teaching in America on the subject. The following paragraph is an attempt at distilling the lesson from the book: 157 years after Lincoln's 1863 Emancipation from Slavery Proclamation: 2018 Unemployment: Black 6.5% White 3.1% has only two explanations: 1.A Racist one: Blacks are no good - you can't help it 2.A Non-Racist one: Some people have done this and we all need to fix it. Now you know what to do if you would like to live in the "Land of the Free."

(1) The Plantation Trilogy by Gwen Bristow, Thomas Y. Crowell, January 1967 edition, in the editorial note titled The Crooked Road