Often in history, the simplest and most obvious answers to basic questions are the correct ones. Although a myriad of interpretations have been offered by historians of the central issues of the US Civil War, in my view the explanations for the causes of the war and the motivations of each side in fighting it are surprisingly straightforward and clear-cut.
The basic advantage the North held at the start of the war, and for the duration of it, lay in its superior manpower and industrial strength. The South had relatively few factories and manufacturing facilities; its economy was primarily based on agriculture. Its ability to fight the war successfully was dependent largely upon its ability to trade with the two European countries which, at the start of the war, were in some sense its de facto allies, Britain and France. Yet the Union initiated a naval blockade of the South which was effective enough to prevent at least partially the unimpeded commerce the South needed to sustain its war effort. And once Lincoln had issued the Emancipation Proclamation, making the cause of abolishing slavery explicit, it became an embarrassment to the European countries to continue supporting the Confederacy.
The South's advantage lay in the enormity of its own territory and in the fact that it needed (or should have needed) merely to fight a defensive war in order to win. The North needed to conquer a land-mass larger than that which the British were unable to subdue 80 years earlier in the War of Independence. The South, presumably, needed only to protect itself from invasion and to keep the war going long enough for the North to exhaust itself and give up the attempt to conquer it.
Neither the North nor the South was monolithic in its motivations for fighting the war. But, contrary to what Southerners themselves and the historians who have acted as apologists for them have asserted, the central reason the South fought to break away from the United States was its objection to Lincoln's plan to prevent the spread of slavery to the territories of the US. The South knew that this, in itself, was an anti-slavery position and, given the fact that many in the North were full-fledged abolitionists who wished to see slavery eliminated entirely, the underlying cause of the secession was to protect slavery. This was made clear when the Confederate Commissioners, the diplomatic agents who were sent from the already seceded states with the purpose of persuading the other states of the South to join them, repeatedly announced the "dangers" the abolition of slavery would pose to Southern society and the consequent need to secede from the Union in order to keep the slave system intact.
The North, at least initially, was on the whole motivated by the desire to keep the Union intact because, if it were to break apart, the cause of freedom and democracy most Americans believed their country had been founded upon would then be weakened or destroyed. The dissolution of the United States would prove that the "experiment" in democracy had failed. But again, the subtext of the effort to quash the secession was the slavery issue. As stated, many in the North were in fact abolitionists. Lincoln himself did not initially believe that slavery should be interfered with in the states where it already existed, but he was nevertheless opposed to slavery, or else he would not have made it his platform that slavery must be prohibited from spreading to the territories. And his wish not to interfere with existing slavery was due to his belief that outright and immediate abolition would be too disruptive to society and would lead to more potential problems than it would solve. With the exclusion of slavery from the territories, the institution, Lincoln and others believed, would "die a natural death." By the autumn of 1862, Lincoln realized that this passive approach was not workable, and he issued the Emancipation Proclamation in January of 1863.
The manner in which the South carried out the war is itself indicative of its wish to defend slavery at all costs. Given the size of its land-mass, as stated, one would think the South merely had to fight a defensive war, using the "Fabian" technique Washington had successfully employed against the British by making strategic retreats and allowing the enemy to take territory but keeping his armies intact in order to fight another day. But for the most part, especially in the actions of their leading general, Robert E. Lee, the Confederates chose to go on the offensive, attacking Union armies repeatedly in order to attempt driving them out of the South (and even taking the war into the Union at Antietam in 1862 and Gettysburg in 1863). The underlying reason for this was that if the Union took territory, it was possible—very possible—that the enslaved people would liberate themselves, even during the early part of the war before the Emancipation Proclamation was issued and the Union commanders did not yet have explicit orders to carry out abolition.
Northern commanders were at first divided on how to prosecute the war. Initially the intention was to aim for a quick victory, end the rebellion, and bring the South back into the fold with little if any disruption to Southern society. This was the thinking of Gen. George McClellan, for instance, and even possibly of a more competent and successful commander such as George Meade. But the strategy did not work. Both Ulysses S. Grant and William T. Sherman by 1864 realized that "total war" was the only realistic method of prosecuting the conflict. Both men succeeded in demoralizing the South through the massive destruction of property—Sherman in the deep South in Georgia and South Carolina, while Grant ordered Gen. Philip Sheridan to destroy the farmland of the Shenandoah Valley in Virginia which had been supplying Lee's army, trapped as it was in Petersburg. The Confederacy, overwhelmed by the superior resources of the Union and by the imminent destruction of the slavery system on which it was built, had no choice but unconditional surrender in the spring of 1865.