Final answer:
The Second Confiscation and Militia Act of 1862 was the Congressional act that intended to void masters' claims to slaves who had worked for the Confederate military, leading to freedom for escaped or captured slaves and authorizing the use of African Americans in federal military service.
Step-by-step explanation:
The act of Congress that sought to void masters' claims to slaves who had been working for the Confederate military was the Second Confiscation and Militia Act of July 17, 1862. This law extended freedom to those enslaved individuals who escaped or were captured by Union armies. It also provided the Union with the power to seize any property used by the Confederacy, including enslaved people. Furthermore, it marked the first official authorization to employ African Americans in federal military service. The act was a step toward emancipation before the official issuance of the Emancipation Proclamation in January 1863. However, it didn't grant former slaves the status of free citizens outright.
Additionally, the Union Congress enacted several other acts to aid formerly enslaved people, such as forming the Freedman's Bureau and the Civil Rights Act of 1866, giving former male slaves the right to vote and to hold public office. These legislative actions were part of the broader efforts to reconstruct the nation's social and legal frameworks during and after the Civil War.