That was surely part (but only part) of his calculation and it worked magnificantly. There were many powerful people in Britain who could see that united, America would eventually eclipse them as the world's leading power and they desired to see the union shattered and weakened and were using their influence in the British government to force them to take the side of the Confederacy. By making the war a moral issue, Lincoln cut the legs out from underneath that faction.
But Lincoln in the summer of 1862 also still hopped that he could break the Confederacy and get at least some of the less radical Southern states to return to the Union using a 'carrot and stick' approach. That, didn't work.
Lincoln was a very, very shrewd man. Not the most popular guy down here, and some of the things he set in motion have largely destroyed our federalist system of government because of unintended consequences, but there’s no denying he was extremely astute politically, and willing to let nothing stand in his way of reuniting the Union—including, in a few cases, the Constitution.
}:-)4