Final answer:
The Civil War dramatically changed the American social, political, and economic landscape, with the most significant effect being the emancipation of enslaved African Americans and the shift towards federal government dominance.
Step-by-step explanation:
The most important effect of the Civil War was the profound and enduring changes it brought to the American society, politics, and economy. Politically, it marked an era where the federal government asserted dominance over the states, an effect that shaped the American political landscape significantly. From an economic perspective, the war devastated the South's plantation system while bolstering the industrial economy of the North, leading to drastic shifts in the nation's economic structure. Socially, the war led to increased tensions between different socioeconomic classes, exemplified by draft and bread riots, and most definitively, it resulted in the emancipation of enslaved African Americans, a monumental step towards equality and a major shift in American social dynamics.