Few former presidents actually managed to survive up to the civil war, and the period from the 1840s to early 1860s was marked with the death of many prominent American politicians. Only 5 were still alive by the war's beginning: James Buchanan, Franklin Pierce, Millard Fillmore, John Tyler, and Martin Van Buren.
James Buchanan, president from 1857-1861 (leaving office only two months before the war began), was opposed to the conflict. In a letter to former asst. postmaster general Horatio King, Buchanan stated "The assault upon Fort Sumter was the commencement of war by the Confederate states, and no alternative was left but to prosecute it with vigor on our part." and would later be heard telling a friend "Every person who has conversed with me knows that I am in favor of sustaining the government in the vigorous prosecution of the war for the restoration of the Union." Buchanan, however, was reluctant to support Lincoln's war policies, generally to prevent himself from admitting the fault of his own leading up to the war.
Martin Van Buren also openly supported the union. In the years leading up to the war, Van Buren fought staunchly to prevent southern secession, if ultimately unsuccessful. He responded probably most harshly of all former presidents to the Confederate bombardment of Fort Sumter: "The attack upon our flag and the capture of Fort Sumter by the Secessionist could be regarded in no other light than as the commencement of a treasonable attempt to overthrow the Federal Government by military force." Van Buren, despite being strongly against slavery, was one of few Democrats not to switch to the Republican party in the 1850's even as the Democratic party began to support the institution. Though initially supporting Stephen Douglass, the 1861 Democratic candidate, the war's onset eventually moved him to support Lincoln. When confronted by Pierce about meeting to use their presidential influence to negotiate an end to the war, Van Buren refused, believing that Pierce's peace terms included allowing the Confederacy's existence to continue. Van Buren died relatively early in 1862 as the conflict ramped up.
Both Millard Filmore and Franklin Pierce denounced secession and initially supported Lincoln's campaign to retake secessionist states through force, however they were far more openly critical of Lincoln's wartime policies than the other ex-presidents. Pierce at one point attempted to call for a meeting of other former presidents to utilize their influence to garner a peace deal, telling his wife "I will never justify, sustain or in any way or to any extent uphold this cruel, heartless, aimless, unnecessary war." Both were critical of the Emancipation Proclamation and Lincoln's suspension of habeus corpus.
John Tyler, unlike the rest, sought to "remain quiet" on the matter of the issues leading up to secession, despite ultimately being a reluctant supporter of it. Even as the Confederate Constitution was being drafted, Tyler joined those who continued to advocate for a compromise with the north that would avoid conflict. Even during South Carolina's vote towards secession, which he supported, he wrote "In the midst of all this I remain quiescent. No longer an actor on the stage of public affairs, I leave to others the settlement of existing disputes... sometimes I think it would be better for all to peaceably separate." Tyler would serve in the Confederate House of Representatives from his election in 1861 until his death in 1862.
Besides Tyler, the former presidents of the time generally agreed that the South should not have seceded and that the north was justified in using its military power to reunite the shattered nation. Even John Tyler was, at best, reluctantly supporting the decisions of his home state.
Buchanan's Civil War, (2005)[^[1]][1] [1]: http://www.lancasterhistory.org/index.php?option=com_content&view=article&id=1239&Itemid=280 "Buchanan's Civil War, (2005)"
Martin Van Buren: The American Presidents Series: The 8th President, 1837-1841, (2005)[^[2]][2] [2]: https://books.google.com/books?id=3G_X-pj1pF4C&pg=PA164#v=onepage&q&f=false "Martin Van Buren: The American Presidents Series: The 8th President, 1837-1841, (2005)"
And Tyler Too: A Biography of John and Julia Gardiner Tyler, (1963)[^[3]][3] [3]: https://archive.org/details/andtylertooabiog013722mbp " And Tyler Too: A Biography of John and Julia Gardiner Tyler, (1963)"
The Expatriation of Franklin Pierce: The Story of a President and the Civil War, (2006)[^[4]][4] [4]: https://books.google.com/books?id=5UoeQZVcG0MC&pg=PA43-IA11#v=onepage&q&f=false " The Expatriation of Franklin Pierce: The Story of a President and the Civil War, (2006)"