This is known as the Anaconda Plan.
This military strategy, developed by Union General Winfield Scott, was meant to starve the Confederacy out of their resources. Essentially, Scott felt that if the Confederacy could not trade for goods with other countries they would not be able to survive. This is due to the Confederacy's reliance on cotton as their main cash crop.
This strategy helps to explain why the US wanted to blockade trade with other countries as well as why the Union wanted to control the Mississippi River, as they knew control of this waterway could negatively impact the Confederacy's ability to transport/obtain resources.
B - Slave revolts.
The revolt of Spartacus, which was also the most famous slave uprising in the history of the Roman Empire, lead to questions about the usefulness and effectiveness of the current governmental system in Rome. This, along with other factors, made them plunge into a crisis which ended through the transition to an empire.
A lack of demand since farm crops and goods were not being purchased as relatively high frequencies during this time
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