In the FAQ.

Most of the Southern slave states which seceded from the Union officially declared defense of the institution of slavery to be one of the main reasons for their secession.

As the secession caused the civil war, and these states had officially declared that secession was to protect slavery, then slavery was 'officially declared' to be the reason for the war before the war even started.

Here is a sample of some of the Southern states secession declarations, which reference protection of slavery as the reason for seceding:

Georgia blames opposition to slavery as the first thing they mention in their secession:

"For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery. They have endeavored to weaken our security, to disturb our domestic peace and tranquility, and persistently refused to comply with their express constitutional obligations to us in reference to that property, and by the use of their power in the Federal Government have striven to deprive us of an equal enjoyment of the common Territories of the Republic."

Mississippi also cites opposition to slavery as the first reason for secession:

"Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin. "

South Carolina first enunciates a claim that they have rights as an independent state, but the first grievance they state is about slavery:

"We affirm that these ends for which this Government was instituted have been defeated, and the Government itself has been made destructive of them by the action of the non-slaveholding States. Those States have assume the right of deciding upon the propriety of our domestic institutions; and have denied the rights of property established in fifteen of the States and recognized by the Constitution; they have denounced as sinful the institution of slavery; they have permitted open establishment among them of societies, whose avowed object is to disturb the peace and to eloign the property of the citizens of other States. They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection. "

Texas declares that the (proposed in Lincoln's platform) ban on the extension of slavery into new territories is their first reason for secession:

"The controlling majority of the Federal Government, under various pretences and disguises, has so administered the same as to exclude the citizens of the Southern States, unless under odious and unconstitutional restrictions, from all the immense territory owned in common by all the States on the Pacific Ocean, for the avowed purpose of acquiring sufficient power in the common government to use it as a means of destroying the institutions of Texas and her sister slaveholding States."

Source: http://sunsite.utk.edu/civil-war/reasons.html

Slavery and perceived threats to the slave system was the common grievance mentioned by these Confederate States.