Why is the union threaten by slavery?

The Union was not threatened by slavery, they thought it was wrong.

ANSWER:

When slavery began in the American Colonies, slaves were also in what would later be considered Northern or Union states of the United States of America.

With the writing, and passage of the Declaration of Independence, slavery slowly began to disappear from colonies in the northern sections.

After the South left the Union, and forced a war on the rest of the United States. It was decided that slavery had to end and be prohibited in the United States forevermore.

The Union wasn't threatened by slavery, but, they didn't want to lose a portion of the territory and people that once belonged to a once united nation.