Tennessee became the 16th state in 1796. It was the last state to join the Confederacy during the American Civil War, and the first readmitted after the war was over, which it was on July 24, 1866. Many of its citizens had remained loyal to the Union, including Andrew Johnson who became US President at the end of the war.
During the Second Great War, however, a US army commanded by General Irving Morrell ran roughshod through the state, capturing the city of Chattanooga with ease. By the end of the war, Tennessee was readmitted to the United States, being the first former Confederate state to be readmitted. Houston and Kentucky were also readmitted to the Union. However, as it was crawling with Freedom Party diehards who had organized a resistance movement, Tennessee would remain under martial law indefinitely.
During the Second World War, Tennessee was the headquarters of a top-secret project to build a new type of bomb powerful enough to level a whole city. However, by 1942, the project had cost millions with nothing to show for it. Herb Druce convinced the government to shut the project down.
In OTL, the project was situated in Oak Ridge, a town on the border of Anderson and Roane counties, created in 1942 for this purpose. It can be assumed that this version of the project is located in the same general region, although the precise details may be different.