Which was the last attempt at compromise before southern secession?

Aside from the firing on Fort Sumter, secession was the last cause in an unfortunate series of events that led the nation to the Civil War. Southern secessionist reasoned that they had the constitutional right to back out of the union that they had only joined as a league of friendship in opposition to the British in the American Revolution. The U.S. Constitution had replaced the articles of Confederation but did not codified that status of states insubordination to the federal government clearly enough to avoid differing interpretations. South Carolina led the way out of the United States and February 1861. Abraham Lincoln said the states had no right to leave the union or to take the mouth of the Mississippi River with them. They six states that followed South Carolina met in Montgomery Alabama to write a new constitution for a new country, the confederate states of America. Lincoln called for 75,000 volunteers to form an army to put down the rebellion in the south.

the main reason that the constitution did not proclaim all men to be free and equal is because of the issue of slavery.