What ever you think, war broke out because of the Union invasion of Virginia. War was initiated by Lincoln on April 16-17 with his declaration of blockade of the South.
So,did the Union invade to stop slavery? Not even Burns makes that assertion.
On April 8, Lincoln notified Gov. Francis Pickens of South Carolina that he would attempt to resupply the fort. The Confederate commander at Charleston, Gen.P.G.T. Beauregard, was ordered by the Confederate government to demand the evacuation of the fort and if refused, to force its evacuation. On April 11, General Beauregard delivered the ultimatum to Anderson, who replied, "Gentlemen, if you do not batter the fort to pieces about us, we shall be starved out in a few days." On direction of the Confederate government in Montgomery, Beauregard notified Anderson that if he would state the time of his evacuation, the Southern forces would hold their fire. Anderson replied that he would evacuate by noon on April 15 unless he received other instructions or additional supplies from his government. (The supply ships were expected before that time.) Told that his answer was unacceptable and that Beauregard would open fire in one hour, Anderson shook the hands of the messengers and said in parting, "If we do not meet again in this world, I hope we may meet in the better one." At 4:30 A.M. on April 12, 1861, 43 Confederate guns in a ring around Fort Sumter began the bombardment that initiated the bloodiest war in American history. http://www.us-civilwar.com/sumter.htm
The above happened before you claim Lincoln "initiated" the war.
As anyone who looks at the facts knows, the official purpose of the war initially was to "restore the union". But clearly secession happened because of the dispute over slavery. Lincoln wasn't even inaugurated until March 5, 1861, and secession was already well under way. The original 7 states had already seceded. You only need to read the speeches of the secessionists to confirm that slavery was the underlying issue.
As for the fact that the Emancipation Proclamation didn't happen for another two years, that was strictly political. Lincoln was a smart enough politician to know that until he won a victory issuing it would seem like the desperation of a losing side. The Battle of Antietam was enough of a victory to allow him to do so. But that doesn't mean that slavery wasn't the root issue behind both secession and the war.