President Abraham Lincoln issued the Emancipation Proclamation on January 1, 1863, as the nation approached its third year of bloody civil war. The proclamation declared "that all persons held as slaves" within the rebellious states "are, and henceforward shall be free."
Despite this expansive wording, the Emancipation Proclamation was limited in many ways. It applied only to states that had seceded from the United States, leaving slavery untouched in the loyal border states. It also expressly exempted parts of the Confederacy (the Southern secessionist states) that had already come under Northern control. Most important, the freedom it promised depended upon Union (United States) military victory
Because Americans felt they were a better society and people than other groups of people (Mexicans/Native Americans) already living out West and somehow that justified them taking over the West. The Mexican government grew frustrated with them moving their illegally.
