Lincoln's version of the causes of the Civil War is perhaps best summarized in his Second Inaugural Address, delivered as the war was winding down in 1865. Harkening back to his first address, Lincoln assessed the situation then:
Both parties deprecated war, but one of them would make war rather than let the nation survive, and the other would accept war rather than let it perish, and the war came.
Lincoln argued, then, that the South, having left the Union, began the Civil War, because their leaving the Union would have destroyed it. He explicitly connected secession to the protection of slavery, claiming that "[a]ll knew that this interest [the slave power] was somehow the cause of the war." So secession was the cause of the war, but underlying secession were the machinations of a slave power that brought it about in an effort to advance their interests.
Abraham Lincoln pointed to the causes of the Civil War in both his first and second inaugural speeches. During the first inaugural speech, the Union was already fractured with seven southern states seceding.
In the speech, Lincoln observed the Southern states unfounded fears with regards to their property, peace and safety. The south attributed these fears to the election of a Republican Administration and a President known to be supporting the emancipation of the slaves.
Lincoln played down these alleged fears and assured the Southern states would continue to enjoy their privileges. However, he warned the seceding states of their breach to the constitution which guaranteed the stability of the Union. He also stated that it was his constitutional obligation to restore and ensure the stability of the Union by all means necessary. Further, suggesting that violence was an option to ensure the seceded states are restored to the Union.
Thus, according to Lincoln, the southern states were in breach of the constitution and were obliged to make amends, failure to which, the national government would be forced to restore the Union.
In your hands, my dissatisfied fellow-countrymen, and not in mine, is the momentous issue of civil war. The Government will not assail you. You can have no conflict without being yourselves the aggressors. You have no oath registered in heaven to destroy the Government, while I shall have the most solemn one to "preserve, protect, and defend it.
In his second speech, Lincoln blamed the Civil War on the South’s desire to continue with slavery. He saw the war as divine punishment for slavery.
Yet, if God wills that it continue, until all the wealth piled by the bond-man’s two hundred and fifty years of unrequited toil shall be sunk, and until every drop of blood drawn with the lash, shall be paid by another drawn with the sword, as was said three thousand years ago, so still it must be said “the judgments of the Lord, are true and righteous altogether.