Final answer:
The main reason many southern states seceded from the Union in 1860 was the fear of federal actions against the South, including slavery abolition. Secession was seen as necessary to protect states' rights and preserve slavery.
Step-by-step explanation:
The main reason many southern states decided to secede from the union in 1860 was the fear that the federal government would take actions against the South, including the abolition of slavery. The election of Abraham Lincoln to the presidency further heightened these concerns.
The southern states believed that secession was necessary to protect their states' rights and preserve the institution of slavery.
Learn more about Secession of southern states from the Union