Lincoln was one of the greatest President. He preserved the Union and banned slavery.
You went to a public school, clearly.
The Union never went away... EVER. Just had fewer members. Period. End of Discussion. If by preserve, you mean he didn't disband the USA after the CSA was formed, then fine, but for four years, the CSA was NOT part of the USA.
And he didn't "ban" slavery. MYTH ALERT. He issued an order to a FOREIGN COUNTRY that had no weight and no bearing on the USA itself.