Complex sentence: Brigadier General Truman Seymour, in command of the expedition landed troops at Jacksonville, in an area already seized by the Union in March 1862, and made several raids into northeast and north-central Florida.
Split into simpler sentences: Brigadier General Truman Seymour, in command of the expedition, landed troops at Jacksonville, in an area already seized by the Union in March 1862. Seymour's forces then made several raids into northeast and north-central Florida.