I wouldn't compare the American Revolution to the Civil War, vis-a-vis your comment on Great Britain. The British were operating at the end of a very long, Trans-Atlantic supply line, while being burdened by the defense of the rest of the Empire (and embroiled, literally, in world war against France, Spain and Holland). The American colonists most certainly did not face the full force of the British Army and Navy. Not quite the same thing as a Northern army enjoying interior lines of supply and the ability to move masses of men and materiel vast distances at speed, due to the extensive rail network and along navigable rivers, mostly across friendly territory (particularly the Mississippi in the later stages of the war).
True, the Union was saddled with mediocre senior commanders, but once the right ones had been found (Grant and Sherman) and the philosophy of total war had been agreed upon, the result was more or less a foregone conclusion. The war, technically, was over when the Mississippi had been closed to the South, and New Orleans taken (most other ports of comparable size having been blockaded by Union Naval power), and it is a tribute to the Confederate army that they continued to fight for as long and as well as they did.
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