Final answer:
George Washington was the young officer who began his military career with the British during the French and Indian War, later becoming a leader in the American Revolution.
Step-by-step explanation:
The young officer who began his military career by fighting for the British in the war was George Washington. Washington's military career began with his involvement in the British forces during the French and Indian War before he became a leading figure in the American Revolutionary War. It's important to note that Washington did not support the British throughout his career; instead, he became a key figure in the American struggle for independence. His leadership was further exemplified when he famously led his troops against the Hessians at Trenton, New Jersey.