Complex sentence: Sawyer began his military career with the United States Marine Corps, but switched to the United States Navy to pursue a career in special operations as a Navy SEAL serving in SEAL Team One and then went on to the Naval Special Warfare Development Group.
Split into simpler sentences: Sawyer began his military career with the U.S. Marine Corps, but switched to the U.S. Navy to pursue a career in special operations as a Navy SEAL serving in SEAL Team One. He then went on to the Naval Special Warfare Development Group.