What do Paul Johnson and Howard Zinn have to say about the civil war?

Paul Johnson asserts that, although the Declaration of Independence stated that the thirteen colonies were at war with Great Britain in 1776, America only became a nation after the Civil War (1861–1865). The word "nation" was not even in the Declaration due to objections from the Southern States. Similarly, the original Constitution of the United States, established in 1787, also made no mention of America as a "nation." In Johnson's A History of the American People, he takes the following quote from a constitutional historian:

The United States has achieved something quite remarkable . . . Americans erected their constitutional roof before they put up their national walls . . . and the Constitution became a substitute for a deeper kind of national identity.

It is also interesting to make note of Johnson's perspective on the connection between slavery and the Civil War. In the same text, Johnson argues that religious forces would have eradicated slavery in America easily, were it not for the importance of cotton, slave labor on plantations, and associated economic wealth.
Howard Zinn discusses Abraham Lincoln's crucial role in the Civil War and the end of slavery in the article attached below. According to Zinn, Lincoln could expertly object to slavery on moral grounds while also employing practical politics to appease the Northern and Southern states. Upon Lincoln's election in 1860, several Southern states seceded from the Union, the Confederacy was formed, and the war began shortly after. In September 1862, Lincoln issued a preliminary Emancipation Proclamation. Although Lincoln personally objected to slavery, Zinn argues:

It was a military move, giving the South four months to stop rebelling, threatening to emancipate their slaves if they continued to fight, promising to leave slavery untouched in states that came over to the North.

Additionally, after the Emancipation Proclamation, blacks were allowed to join the Union army. Zinn argued that, as more blacks entered the war, the more it appeared that the war was for their liberation. Shortly after the Civil War in 1865, the Thirteenth Amendment outlawed slavery.
Of course, Zinn and Johnson have much more to say about the Civil War; the perspectives I outlined are snapshots, so to speak, of specific interpretations related to the conflict.
https://libcom.org/history/lincoln-emancipation-howard-zinn