Complex sentence: Only the day before Indianapolis's citizens were celebrating the Union victories at Vicksburg, Mississippi, and Gettysburg, Pennsylvania, but the city's mood turned to panic, when Morgan's troops appeared to be headed toward Indianapolis.
Split into simpler sentences: Only the day before, Indianapolis's citizens were celebrating the Union victories at Vicksburg, Mississippi, and Gettysburg, Pennsylvania. The city's mood turned to panic when Morgan's troops appeared to be headed toward Indianapolis.