Final answer:
A. Western; Mississippi River; Eastern states. The Union aimed to control the Mississippi River in the Western United States while both the Confederacy and Union fought for dominance in the Eastern states during the Civil War.
Step-by-step explanation:
Fighting the Civil War spread to the Western United States. Union strategy in the West centered on control of the Mississippi River. Confederate and Union troops struggled for dominance in the Eastern states.
During the Civil War, multiple strategic objectives were pursued by Union forces in different regions of the United States. In the Western theater of war, the Union sought to take control of the Mississippi River, which was crucial for logistic and strategic reasons. Control of the Mississippi River would cut off supplies and split the Confederacy into two, which was a significant objective of the Anaconda Plan, the Union’s strategic outline to defeat the Confederacy. Ulysses S. Grant's Army of the West played a pivotal role in this strategy with victories in the crucial battles such as at Shiloh and eventually at Vicksburg. Meanwhile, military engagements in the Eastern states included significant battles like Gettysburg, with troops in this region often referred to as the Army of the Potomac. The control over the Western and Eastern theaters was seen as essential to the overall outcome of the war.