The one of the reason's why the Allies allied themselves with the soviets was because they wanted to open a new front. The Soviet Front would be so massive and would force Axis's units to spread even more, and to spread resources more. Though they were already spread out.
Answer:
Before things like mountains were typically boundaries since they were a pain in the butt to cross. But with modern planes, and depending on the mountian, cars, mountains aren't as big a pain anymore. Deserts might have also been a boundary before. Today you turn on the AC and drive straight across it. At least for technologically advanced western countries, there really isn't any geographic feature we can't easily get around.
Southern slaveholders did not want the Congress to take the power over, so slave trade while controlling foreign affairs. So they only let the Congress have power over trade, excluding slave trade, for next twenty years.
