Answer:
Economic warfare
Step-by-step explanation:
Economic warfare is a military strategy in winning the war against the opponent. It involves the use of economic policies such as the blockade of resources to the enemy's side, seizing and preventing the supply of vital economic resources of the opponents such that the opponents can not fight at full gear or strength.
Hence, For the Union to win the war, they had to strike at the enemy's economic resources. This was called ECONOMIC WARFARE