Final answer:
The Black Codes, laws aimed at maintaining white supremacy by restricting the rights of African Americans, led Congress to pass the Civil Rights Act of 1866 to protect civil liberties and establish African American citizenship.
Step-by-step explanation:
The event that spurred Congress to pass the Civil Rights Act in April 1866 was the enactment of the Black Codes by the former Confederate states. These restrictive laws were designed to maintain white supremacy and severely limit the freedom and rights of African Americans, who were recently emancipated following the Civil War. In response to the Black Codes and the clear intent of southern states to undermine civil liberties, the Congressional Joint Committee on Reconstruction took action. The Civil Rights Act established citizenship for African Americans and granted the federal government the power to intervene in state affairs to protect citizens' rights, contradicting the Dred Scott decision. Despite being vetoed by President Andrew Johnson, Congress overrode the veto, marking a significant legislative achievement in American civil rights history.