My gg-grandfather joined the Union Army in New Orleans in Nov 1864, was assigned to Capt JJ Smith's 1st Company Pontooners U.S. Volunteers. He was treated for a medical problem in Alexandria, La prior to Nov 1865. In that Pontoonier units were assigned to marching Armies, how can I determine what Army his unit was assigned to? What action was occuring in Alexandria to warrant Union medical facilities?