Answer: Abraham Lincoln wanted to focus on preserving the Union instead of enslavement.
Explanation: In Lincoln's letter to Horace Greely, he states "if I could save it [the Union] by freeing some [slaves] and leaving others alone I would also do that."
"<span>Native American slaves were replaced by African slaves because of treaties signed between the Europeans and the Native Americans" is the best choice but the Natives also knew the land too well and often escaped. </span>
