After the EMP and the major cities were destroyed by the Espheni, the government of the United States collapsed and the people were left to fend for themselves. Some of the remaining military personnel began consolidating all the people and supplies they could to form a resistance.
At some point Dr. Arthur Manchester arrived in Charleston, South Carolina and began bringing people together. Much of the former living conditions were able to be restored. After an agreement with Dr. Roger Kadar, Charleston was able to get restore electricity to a few square blocks of Charleston. A new government was formed under the Continental Congress and the military was united under the 1st Continental Army.
The first president of the New United States was Tom Mason. Although Arthur Manchester lead the city of Charleston before him, he was not the official president. Tom Mason was succeeded by Marina Peralta and she is currently the president of the New United States.
23 days after the Second Assault on Boston, the Espheni attack all over the world in force and Charleston becomes an Espheni Ghetto. Presumably the New United States is destroyed with it as its not seen or mentioned again.