The Union needed to capture Confederate territory, but the Confederates didn't need or want to invade Union territory.
Again I ask please watch this video, I'd say it is on a Junior High level so you will be able to keep up.
Sure, well... except for Missouri, Kentucky, West Virginia, Oklahoma & New Mexico/Arizona -- claimed by Confederates.
Well... also Maryland, Pennsylvania, Ohio, Indiana & Kansas, invaded by Confederates and driven out by Union forces.
Then there were Confederate guerillas in California, Colorado & Vermont, among others.
Indeed, when you count them up, in the Civil War's first 12 months Confederates fought more battles and lost more lives in the Union than in the Confederacy.
But I "get" that all that was just defense, DE-FENSE!, I mean, who needs stinkin' offense anyway?