According to the Tenth Amendment all powers that were not delegated to the government of the United States that are not prohibited to the states are reserved to the states and to the people.
The states have the right to do whatever they wish as long as the action is not prohibited to them.
The right of state to have slaves is a right guaranteed to them in the Constitution and the right of states to secede from the Union in not prohibited to them.
The only way slavery could have been lawfully abolished is by a Constitutional Amendment and is was highly unlikely that three-fourths of the states would have supported the abolition of slavery. At the
At the outset of the Civil War in fifteen of the thirty-four states slavery was legal and nineteen states it was not. In order to abolish slavery six of the fifteen slave states would have been needed to votes for its abolition.
Delaware, Maryland, Missouri and Kentucky may have joined the 19 free states to vote for the abolition of slavery This would have given the opponents of slavery 23 votes for a proposed amendment to ban slavery.
In order to obtain the requisite number of votes at least two addition slave states would have been needed to abolish slavery without having to fight the Civil War.