sentences:  Grant and Generals Hooker, George H. Thomas, and William Tecumseh Sherman led the Union to another major victory at the Third Battle of Chattanooga in November, driving Confederate forces out of Tennessee.The capture of Chattanooga left Georgia vulnerable to attack, raising the possibility of a Union march to the Atlantic Ocean, which would divide the Confederacy.In April 1861, Lincoln announced the Union blockade of all Southern ports; commercial ships could not get insurance and regular traffic ended.