The disunion forever: Why the US Civil War still divides America 150 years on

At 4.30 on the morning of April 12 1861 – 150 years ago this week – the newly-formed Confederate States of America opened fire on Fort Sumter, located near the entrance to Charleston Harbor, South Carolina and held by troops loyal to the Union. This was the spark for conflict, the scale of which was glimpsed by virtually no-one in 1861. By war's end four years later, some 620,000 Americans would be dead.

Why had the war broken out? This question continues to divide Americans, even today. Some continue to defend the Confederacy, while others will not hear a word of criticism of the Union. Even to point out that either side made mistakes of judgement or of fact is to court unpopularity.

Much of the controversy centres on the role of slavery in the conflict. An overwhelming majority of scholars, but not necessarily of Americans as a whole, agree that slavery was the central cause of the conflict. In 1776 slavery had existed throughout the newly-formed nation, but it had been gradually and painlessly abolished in the Northern states. Many had expected the South to go the same way, especially after the supply of slaves from Africa had been cut off in 1808.

But this had not happened, partly because of the almost insatiable demand for the crops produced in the South by the slaves, with cotton of course the most important of them.

The Confederacy's modern defenders often do not seek to defend slavery; instead they argue that slavery was not the central issue of the Civil War. And it is true that there were other causal factors at work. For example, Southerners were wont to emphasise the rights of the states (in opposition to federal power) and modern day neo-Confederates, seeking also to roll back the federal government, find much to admire in their spirit. This attitude resonates with a much wider American hostility to government: Confederates are seen as having been faithful to American libertarian traditions. Although sympathy for the Confederacy is by no means a general characteristic of the modern Tea Party movement which, of course, harks back to the American Revolution rather than to the Civil War, some Tea Party-goers clearly do find much to admire in the Confederacy. Some of them even argue that although in Lincoln the nation elected a Republican he was, in fact, the wrong kind of Republican, one insufficiently committed to low taxes and a weak federal government. Similarly, the many Americans who display the Confederate flag on their car or truck bumpers are not usually applauding slavery, but are instead either affirming a hostility to centralised power or simply expressing a vaguer rebelliousness of spirit.

Lincoln's election in 1860 as the first Republican president, was the trigger for secession. But rather than being driven by a humanitarian concern for the slave, modern Confederate sympathisers argue the Republicans were motivated by a thirst for economic and political power. They reckon that Lincoln and his party wanted to strengthen the political and economic power of the North, at the expense of the South. Northerners, they say, were racists, like Southerners.

There is some truth in these claims. By modern standards, Abraham Lincoln and the vast majority of his supporters were indeed racists: they did not believe that African Americans were, or should be, equal to whites.

Few Republicans in 1861 believed it possible to free the slaves of the South in the immediate future. But almost all of them, whether or not they had the welfare of the slave at heart, believed that the North was politically and economically superior to the South and that it was time to place in the White House someone who would promote Northern interests and values.

But for all this, neo-Confederate claims about Lincoln and his party are highly misleading. The Republicans, with Lincoln at their head, displayed a complex set of attitudes, which defy easy generalisation. But slavery was at the very centre of the conflict between North and South. Even though Southerners did display a concern for the rights of the states and a hostility to federal power, this was usually in defence of slavery. And when that defence seemed to require an extension of federal power, Southerners would quickly set aside their inhibitions about the Big State. For example, in 1850 Congress passed a new Fugitive Slave law which required all citizens, North and South, to come to the aid of the slaveholder (or his representative) who was pursuing a fugitive slave.

It was now Northerners who objected to this extension of federal power and who accordingly rallied to the defence of state's rights.

It was not primarily the working conditions of the slave, with a lack of pecuniary reward and a vulnerability to violence that most incensed anti-slavery crusaders. Instead it was the absence of legalised marriage and the possibility that families might be broken up at the whim of the master.

According to Harriet Beecher Stowe, author of Uncle Tom's Cabin, the greatest work of anti-slavery propaganda of the 19th century: "The worst abuse of the system of slavery is its outrage upon the family."

Even though he did not believe blacks to be the equal of whites, Lincoln always believed that slavery was a grievous wrong. But this conviction did not lead him to seek its immediate abolition. Instead he wanted slavery to be "put in course of ultimate extinction". One way of achieving this was by guaranteeing that there would be no new slave states. As the nation spread out to the West, the question of the status of slavery in these new states became urgent. The Republican party was not committed to immediate abolition but it was committed to preventing its spread.

Alongside the moral criticisms of slavery, which not all Republicans endorsed, was a set of political and economic criticisms which virtually all Republicans did accept.

One claim was that slavery retarded an economy's growth and development. On the eve of the Civil War the South lagged far behind the North in many of the indices of economic and especially industrial development.

In the growth of towns and cities, in the development of manufacturing industry, in banking provision, in road, railroad and canal mileage, the North was far ahead. Republicans unhesitatingly attributed this to the evil effect of slavery which, it was claimed, destroyed incentives. Similarly Lincoln and the Republicans believed that the slaveholders posed a profound threat to the nation's democratic institutions and values. Only a minority of Southern families owned slaves, but the wealth conferred upon this minority, according to Republicans, meant that they formed an aristocracy that ruled the South with a rod of iron and that was trying to achieve similar control over the nation as a whole. A Republican victory was seen by many in the party as a deliverance – a step toward breaking the power of this Southern aristocracy.

It is vitally important to note that this political critique of slavery, like the economic criticisms, could be, and was, adopted by some in the Republican party who were not at all concerned with the welfare of the slave. These Republicans were motivated instead by a desire to further the interest of white Americans, whose democratic rights and economic opportunities, they held, were under threat from the Southern elites.

So slavery was indeed at the heart of the conflict, though it was not opposed in the North exclusively or even primarily on moral grounds. When the war started, Lincoln was committed not to emancipation but instead to maintaining the Union, coercing those slaveholding states that had left the Union into rejoining it. Not all the slaveholding states even joined the Confederacy; the four where slavery was weakest (Delaware, Maryland, Missouri and Kentucky) remained in the Union (though many of the citizens of these states in fact volunteered for the Confederate armies). At the beginning of the war, Lincoln was acutely conscious of the need to retain the support of these states. But this does not dispel the claim that slavery was at the heart of the conflict. The Confederates made it clear that this was the reason they had left the Union; only after the war was over, did they begin to claim otherwise. In the slaveholding states that did not secede, the slaveholders simply lacked the power to break the ties that bound their states to the Union.

Even when the war began in 1861, the seeds of emancipation were present in the Republican's stance on slavery and these convictions would bear fruit in 1862 with the Emancipation Proclamation.

But the racist assumptions that were so widespread even in the North would, over the coming decades, do much to undermine the freedom that that Proclamation offered to more than three million African Americans.

It may be 150 years since the Civil War began, but the triumphs and failures of that extraordinary conflict continue to reverberate.

The Civil War endures.

John Ashworth is Professor of American History at the University of Nottingham and the author of many books and articles on the American Civil War