Difficult to define, a moving target, it is where the western armies fought...all comments from posts on the Facebook group. Ask a Civil War buff and the answer may be simple, or might not be simple. For me, it is more geographic than army based, but even my simplification is liable to be picked apart. Using the Allegheny Mountains, Mississippi River, and the eastern line of the Rocky Mountains, my interpretation looks like this:
If we use an army based approach, say for course of study the Confederate Army of Mississippi/Tennessee, then technically the Trans-Mississippi Theater could be the Western Theater because many units that were formed or saw service in the Trans-Miss would later be morphed into the aforementioned Confederate army. Conversely, units under Van Dorn and Price that were really part of the forces in the Trans-Miss fought at Corinth, Mississippi in October 1862, so is the Western Theater really the Trans-Miss? And what about the Gulf or coastal regions - how do they become defined?
Looking at geographic features, the Allegheny Mountains divides the Eastern and Western Theaters, while the Mississippi is the dividing point between the Western and the Trans-Miss Theaters. But as the Allegheny Mountains peter out northeast of Atlanta, do the remaining portions of Georgia become part of the west or are they in the east? And the Mississippi is not the dividing line is seems to be, at least in the following interpretation by the National Park Service. Belmont and locations west of Vicksburg are part of the Western Theater. The NPS also gives us a fifth theater, the Seaboard/Gulf, muddying the waters even more.
What then is this Pacific Theater and why does it extend hundreds of miles inland? Seaboard/Gulf region? Wouldn't Brownsville, Texas be part of the Seaboard/Gulf instead of the Trans-Miss? And it is apparent that the NPS considers Sherman's operations in North Carolina as part of the Eastern Theater. More confusion, less agreement.
We all have our own thoughts about the Western Theater's expanse, and geographically it is not that easy to clearly define. So therefore I ask the readers to leave a comment here with your own ideas as to what the Western Theater is in your opinion.
From an operational standpoint, the Eastern theater should be defined as VA/PA and part of NC. The Western theater should run from the western drainage of the Appalachians, at least through WV, but East Tenn and Cumberland Gap belong in the Western theater, so the line bending into mid NC is appropriate. The Gulf/Atlantic seaboard is also a completely different theater: troops are sent to and from these locations but they do not conduct mutually supporting combined operations between theaters.
The Western Theater's western boundary is more complicated. It clearly should encompass the entire Mississippi Valley, at least as far as Port Hudson, where I'd argue that the Gulf/Atlantic theater takes over. This means that Grant's operations west of the…
For me, it has always been difficult to define, due to the constant overlap of forces transcending several of the natural boundaries. I used to think that wherever the main armies of that theater were located, then that was the theater. But...when I started to look more into it, you have several Eastern Theater units make their way from the Army of the Potomac to Chattanooga in 1863. The theater didn't change, but the make up of the forces did. Like you said, Van Dorn eventually crossed the Mississippi in 1862. The Trans-Miss didn't follow him. If Sherman had been able to reach Virginia before Lee surrendered, I don't think we would consider that to be anything but a Western…