Complex sentence: In most cases, career foreign service officers serve a tour of approximately three years per ambassadorship whereas political appointees customarily tender their resignations upon the inauguration of a new president.
Split into simpler sentences: In most cases, U.S. ambassadors who are career foreign service officers serve a tour of approximately three years in a foreign post. Ambassadors who are political appointees will customarily tender their resignations upon inauguration of a new President.