Preserving the Constitution and the rule of law: The United States Constitution established a federal government that derived its power from the consent of the governed. The Southern states, by seceding from the Union, were challenging this principle and threatening to undermine the very foundation of the nation. Preserving the Union was seen as essential to upholding the Constitution and maintaining the rule of law.
Economic reasons: The Union had a much stronger economy than the Confederacy, with more industrial capacity, a larger population, and access to more resources. Secession would have meant the loss of valuable Southern agricultural exports and the disruption of trade and commerce between the North and the South, resulting in severe economic consequences for both sides.
Political power and influence: The Southern states had a disproportionate amount of political power in the federal government due to the Three-Fifths Compromise, which counted slaves as three-fifths of a person for purposes of representation in the House of Representatives. Secession would have shifted the balance of power in favor of the Northern states, which were opposed to the expansion of slavery.
Fear of a larger conflict: Many Northerners feared that secession could lead to a wider conflict that might involve foreign powers and jeopardize the stability and security of the entire nation. They believed that preserving the Union was necessary to prevent a costly and destructive civil war.
Moral and ideological reasons: The abolitionist movement in the North was gaining momentum, and many Northerners opposed the institution of slavery on moral grounds. They believed that the Union should not allow the expansion of slavery into the Western territories and that it had a moral obligation to end the practice altogether.
In summary, the Union needed to stay together to uphold the Constitution, maintain its economic strength, preserve its political power, avoid a larger conflict, and address the moral and ideological concerns of its citizens.