Eastern Theater of Operations
The Eastern Theater of Operations is generally thought to include from north to south, just north above Harrisburg, Pennsylvania down to the Virginia/North Carolina border, some even limit that to just south of Petersburg, Virginia. East to west, from the Atlantic coastline to the western borders of current day West Virginia and Virginia, this would include most of the Shenandoah Valley, Washington D.C., Maryland and Delaware.
For our purposes we will use the geographical landmarks that the government used to divide the areas of operation at the beginning of the war. This would make the Eastern Theater of Operations, North to South from the top of Pennsylvania to the bottom of Florida (some include Florida into the Gulf region). East to West from the Atlantic coastline to the middle crest of the Appalachian Mountains. Where the mountain range ends on the south-west edge the Eastern Theater of Operations would border the eastern edge of Alabama running through the upper north-west area of Georgia (near Dalton).
This lower area became fluid as the Western Armies of Sherman began taking Georgia, South Carolina and finally North Carolina in 1864 and 1865, our way of looking at it was just that that Western Armies were operating in the Eastern Theater of Operations and had left the Western Theater of Operations when they started their campaign against Atlanta.
The Eastern Theater of Operations also contained, after 8 May 1861, both the United States Capital in Washington D.C. and the Rebel Capital in Richmond, Virginia. This close proximity created quite a chess match between the two forces for four years until the breakthrough of Petersburg began the end to the war.