A point I found interesting along the lines of perceived British ambivalence, my understanding is that the Emancipation Proclamation was a brilliant piece of diplomacy. It was anti-slavery enough to make it possible to convince Britain that the war was all about slavery and so they shouldn't intervene because of how much they were against slavery. Yet also weak enough against slavery - only freeing slaves in the territory they hadn't conquered yet - to make it possible to convince the Union officers and politicians that it was a tactical economic ploy against the Confederacy and not a changing of the primary war aims. They were rather ambivalent about slavery, and Lincoln himself feared that "half the officers would fling down their arms and three more states would rise" if he made full emancipation a primary war aim.