Explain why Lincoln decided to issue the Emancipation Proclamation and how it impacted the war.

Lincoln decided to issue the Emancipation Proclamation only after a lot of consideration.  Lincoln did not want to alienate the border states, particularly Missouri, Kentucky, and Maryland, who still owned slaves.  If these states went to the Southern camp, it would provide a large morale boost to the Confederacy and present a border that was harder to defend for the North.  It would also surround Washington D.C. with hostile territory.  Lincoln only freed the slaves in the Confederate states.  Lincoln wanted to curry international favor by issuing the Emancipation; Britain had already ended slavery in its colonies and was quite active in ending the global slave trade.  Lincoln also wanted to placate abolitionists back home, though right after the Proclamation went into effect there was some discontent in the Union army when some soldiers said that they did not sign up to free slaves.  Lincoln also wanted to give slaves in the South incentive to escape; the prospect of slaves escaping might lead to more Southerners going home to manage their plantations thereby weakening the Confederate armies.