- Sep 1 6th Ohio Cavalry ambush at Barbees Crossroads Virginia
- Sep 1 Federal troops reconquer Fort Smith Arkansas
- Sep 1 Rail & ferry connection between San Francisco & Oakland inaugurated
- Sep 5 Bread revolt in Mobile, Alabama
- Sep 6 -7] After 59 day siege Confederate troops vacate Fort Wagner SC (1700 casualties)
- Sep 7 Federal naval expedition arrives off Sabine Pass
- Sep 8 -10] Federal troops reconquer the Cumperland Gap, Tennessee
- Sep 8 Battle of Sabine Pass TX: 47 Texas volunteers repel Federal forces
- Sep 8 Battle of Telford's Depot TN
- Sep 9 Battle of Cumberland Gap, TN
- Sep 16 Robert College of Istanbul, Turkey, the first American educational institution outside the United States, is founded by Christopher Robert, an American philanthropist.
- Sep 17 Pope Pius IX encyclical On persecution in New Grenada
Battle of Interest
Sep 19 Battle of Chickamauga, GA (near Chattanooga) begins; Union forces retreat after a couple days of fighting
Gordon Granger
- Sep 20 Battle of Shepardstown VA
- Sep 20 US Civil War Battle of Chickamauga, near Chattanooga, Tennessee ends with a Union withdrawal
- Sep 21 Union forces retreat to Chattanooga after defeat at Chickamauga
- Sep 23 Confederate siege of Chattanooga begins
- Sep 27 Jo Shelby's cavalry in action at Moffat's Station, Arkansas