Caliban and Ariel, because they hate each other even though they are both slaves.
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Landowners often took advantage of workers
Home-ground advantage - along with the stronger war-mission that went with it, driving out the invader.
The North's biggest advantage was its industrial might. The rural and agricultural South couldn't keep up.
One advantage was their superior industrial capacity, but the North also had a much larger population with which to replenish their losses.
In the US Civil War, the North had the advantage in population and industry. The South had the advantage of better officers, and that they were defending their homes.