The answer would be: “Free African American(s) were allowed to enlist in the Union army and Navy.
If they wanted to fight against the confederacy and the slavery supporters then they were allowed to because the Emancipation Proclamation allowed them to join the Union army.
Hope this helps. :)
Because they want to keep their european Rivals Away from Valuable town and Mines in Mexico.
North America was really Loaded with Gold and Silver, and has Accounted for more than 20 % of Spains Budget back then. That's why they want to assert control over it