why did the north resist secession in the 18th century?

Answer 1

got me

Answer 2

The North did not RESIST secession, but enforced the laws of the Constitution of the U.S. "...all men are created equal..."

As time went on and more States entered the Union, they chose either to become a "Slave State" or a "Free State". Ultimately this schism caused problems, not only between States, but WITHIN them, as well. The Southern States seceeded because they wanted to maintain their way of life, including the most important aspect to their way of life, slave ownership.

So... when Abe Lincoln signed the Emancipation Proclamation into law, it meant that Slavery was now ILLEGAL. The South couldn't deal with that, so they tried to start their own country. This is what started the American Civil War. Many prior pieces of legislation made secession impractical. The Monroe Doctrine went the other way and prohibited other countries from colonizing on North American soil.