Final answer:
Congress passed a nationalistic economic program during the Civil War primarily because Southern opposition was absent due to secession. They levied taxes, set high tariffs, enacted the National Bank Acts, and introduced greenbacks to finance the war, which led to Northern economic growth.
Step-by-step explanation:
The circumstance that enabled Congress to pass a nationalistic economic program during the Civil War was the absence of Southern opposition in Congress due to the secession of the Southern states. With Southerners gone, northern legislators could freely pass measures that favored the northern economic agenda. To finance the war effort and foster economic growth, Congress implemented taxes on income and inheritances, established high tariffs, and passed the National Bank Acts of 1863 and 1864. These acts encouraged the purchase of war bonds and the issuance of greenbacks, which resulted in economic boom in the North, despite the rise of inflation. Moreover, the Legal Tender Act of 1862 established the greenbacks as legal tender, facilitating transactions and the government's ability to mobilize financial resources for the war.