The Civil War was fought to establish that point.
Most didn't, for obvious reasons. Do you mean 'Which slave-states did not leave the Union?' They were Missouri, Kentucky, Maryland and Delaware.
Because Lincoln had won the 1860 election on a ticket of no new slave-states.
There were five slave states that remained in the Union. Initially there were four -- Delaware, Maryland, Kentucky and Missouri. West Virgina separated from Virginia when it (Virginia) seceded from the Union. West Virginia was admitted to the Union in 1863 as a slave state. West Virgina remained in the Union making it the fifth slave state not to secede. These five slave states were called border states.
Maryland, Delaware, Missouri, and Kentucky. Lincoln was worried that forcing them to release their slaves at this point might cause them to leave the Union and join the Confederacy.
Because Lincoln had won the election, on a promise of no new slave-states. So the South would always be outvoted in Congress.
West Virginia broke apart from the slave state Virginia and joined the Union, but for the most part all slave states were Confederate.
Virginia was a Confederate state. The Union slave states were Maryland, Delaware, Missouri, and Kentucky. In June 1863 West Virginia became the fifth Union slave state.
union. It was one of the four slave states in the Union
Weat Virginia Delaware, Maryland, Missouri, and Kentucky were five slave states that did not secede the Union, and the were were called border states.
The Union (Northern states)
The northern states did want Missouri admitted to the union as a slave state so as to balance the free slave trade.