The 14th Amendment overturned the Dred Scott decision. This amendment gave citizenship to more than 4 million previously enslaved African Americans. The 14th Amendment was part of the Reconstruction Amendments, which included the 13th (abolished slavery) and the 15th (gave African American men the right to vote). These amendments were passed between 1865-1870.
The United States of America joined World War II when Pearl Harbor was bombed...if you're referring to the 2nd World War.