What was the main reason Lincoln issued the Emancipation Proclamation?

Lincoln's motives in issuing the Emancipation Proclamation were complex. As is well known, the wording of the document meant that it did not really free enslaved people. It only applied to slaves held in states "in Rebellion against the United States," areas where he had no political power. In the border states, and even in some southern areas occupied by federal troops, the Proclamation did not free slaves. While there was a moral dimension to the Emancipation, it was fundamentally a wartime measure, a political document. The best way, perhaps, to describe Lincoln's main motive in issuing the Proclamation is to say that he intended it to redefine the purpose of the war. Lincoln had, since the beginning of the war, maintained that it was a war to preserve the Union. The Emancipation Proclamation ensured that preservation of the Union was incompatible with preserving the institution of slavery. This was especially important when considering the global diplomatic context of the war. If the war was fundamentally about slavery, Great Britain (which had abolished slavery) would not intervene to recognize the Confederacy as Southern leaders fervently hoped they would. Lincoln seems to have genuinely believed by this point that the war should mark the end of slavery in the Confederacy, and the political and strategic benefits to this position strengthened this belief. The Emancipation Proclamation ensured that this would happen.