Why is the American Civil War sometimes referred to as the War of Northern Aggression?

The American Civil War is sometimes called, and even taught, as the War of Northern Aggression. There are two main reasons for this; the first and more simple answer is that during the Post-Reconstruction Era, a lot of the projects designed to give former slaves equal civil rights under the fourteenth and fifteenth Amendments to the Constitution collapsed. Racist segregation policies and later Jim Crow laws were enacted across the states, and especially in the South. This included segregated schooling and therefore the legacy of the Civil War was changed over time, from a war fought in defence of slavery, to a war fought in defence of states rights, and against the encroachment of northern politicians. This is a fairly simple explanation; it is a lot easier to erect statues of and teach about men that could be classified as heroes of your state, than it would be to celebrate them in their defence of slavery. The second and more complex answer reflects the fundamental tensions between federal and states rights that existed from the beginnings of the new nation. From the Declaration of Independence, there was a tension between those who believed in States rights, and those who wanted to strengthen the federal or central government. The states originally declared their independence from Britain as thirteen separate colonies, united only in their desire to be challenge British tax laws. The original constitution, the Articles of Confederation, gave very little power to a centralised government. The revised Constitution written at the Constitutional Convention in Philadelphia significantly strengthened the role of the federal government, including giving them the right to collect income tax. As the South became increasingly dependent on the cotton plantation economy whilst the north diversified into manufacturing throughout the Antebellum period, states rights became an increasingly difficult topic. America was expanding rapidly, adding more states that, after the 1821 Missouri Compromise, were admitted in a balance between slave and free. By the mid-1840s, the question of slave and free was dividing the nation, and many Democrats, including those in the North, believed that parties like the Free Soil Party and the newly formed Republican Party were going to attack their constitutional 'right' to own slaves as soon as they got into power. Southern slave owners had long held a relative monopoly over Congress, but the balance of power was shifting. Lincoln winning the election of 1860 confirmed their fears, and prompted the Winter of Secession. Some of those Democrats therefore called it the War of Northern Aggression because they saw themselves as the true defenders of the Constitution and of the rights of states to choose their own policies, and not be dictated to at a federal level. Some, like Confederate general Robert E Lee, were very torn between their loyalty to their state and to their country, but believed that the federal government would go too far in trying to indirectly end slavery, and that therefore they had to fight on behalf of the South not in defence of slavery, but in defence of their own state and home.