I think some reasons for the American Civil War are 1) desire of northern industrialists and politicians for empire, 2) desire of the Republican Party for political hegemony, 3) idealism: many Northerners had a vision for America that did not include state rights, slavery, or differences in sections as to life styles. A few Northerners were fanatical in their support of this vision and in their opposition to the differences manifested by the South. 4) Northern industrial interests supported the war because they wanted access to the labor force that was enslaved in the South, and they could have that access only if the slaves were freed. I don't know that they went to war for this reason, but they definitely saw this as a reason to support the war once it had started. 5) a lot of Southern planters were in debt to Northern capitalists. If the South seceeded, the Northern Capitalists might not be able to collect their debts. Again, I don't know that they went to war for this reason, but they definitely saw this as a reason to support the war.
In the halls of Congress, mercantile and industrial interests and politicians from states where these interests held hegemony (the North), wanted to pass laws to tax the people for support of programs that would subsidize mercantile and industrial interests. They wanted government to improve the harbors and rivers that they used. They wanted government to build railroads for their use. They wanted protective tariffs so they could charge more for their products. Agricultural people opposed most of these measures because they did not want to pay the additional taxes for support of commerce and industry. The slave-states were almost completely agricultural with very little industry. As long as slave states and free states were equal in number, they each sent the same number of senators to Washington. The slave states, having the same number of Senators, could defeat a lot of the expensive programs for subsidizing industry and commerce. When the Republican Party moved to block slaves from the territories so that no more slave-states would be created, the slave-states saw that they would become mere colonies of the free-states.
This of course is not a reason for war; rather, this is a reason for secession. The reason for war is that the North and its politicians were bent upon empire. (They would soon bring under their control, the South, the Carribean, much of Latin America, Hawaii, the Philipines, and part of China.) Letting the South seceed would reduce their empire just as they were starting to grow it. The Republican Party would have lost the next election, if the South successfully seceeded, because northern voters would have seen the break-up of the Union as a failure of the Republican Party. For these reasons, President Lincoln launched an invasion of the South to prevent secession. These were the reasons for war.
The common men in the North who supported the war, did so because their flag had been fired upon at Ft. Sumper.
The commen men in the South who supported the war, did so because their home and hearth was invaded by hostile military forces. (Lincoln launched an invasion fleet from New York to relieve Ft. Sumter; the South attacked Ft. Sumter to prevent that fleet from landing.)
The argument that secession caused the war is without foundation. A number of the divisions of the Soviet Russian empire seceeded in the 1990s without war. When the American states entered the Union, some of them did so with the expressly stated stipulation that they did so voluntarily and could withdraw voluntarily; the Constitution does not say otherwise, for if it did so, the thirteen original free and independent states would not have joined. The U.S. Supreme Court decided in favor of empire over free government only after the American War of 1861-1865. Before the War, secession was considered a right by the vast majority of people. The threat of secession must be available so that minorities can extract political compromises from the ruling majority; without the threat of secession, they are but subject peoples, not free and independent. Of course, after the War, the party that had used illegality of secession as an excuse to prosecute the War, made a big argument that it was illegal. The victor gets to write the history and might makes right, but