The two strategies used by the Union during the Civil War were:
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Naval blockade - The Union enforced a naval blockade of Confederate ports to cut off supplies and trade with Europe and within the Confederacy.
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Seizing control of the Mississippi River - The Union aimed to gain control of the Mississippi River to divide the Confederacy and secure key transportation and supply routes.
The other options, defensive war (which was primarily a strategy of the Confederacy) and relying on European countries for supplies (which was not a primary Union strategy), do not apply to the Union's military approaches.