What is Lincoln's main about the union

I'm guessing you mean what was Lincoln's main goal for the Union. It will be that Lincoln wanted the Union to stay together and abolish slavery.

Explanation:

Because of disputes between the Northern and the Southern colonies over the issue of slavery, the South had decided to secede from the Union (it was what the United States was called before) and just have it's own country. Britain, who once ruled the colonies, was excited about this idea. Probably because they thought they would be able to seize the colonies back. This really frustrated Lincoln, especially since barely a century ago the country had liberated themselves from Britain. He pushed for slaves to be free, and he went into a tiring 5 year war (Civil War) to keep the country together.