Complex sentence: Kerry "had" served in Vietnam, but became better known, and somewhat controversial, after his discharge for speech and testimony opposing the war and implicating fellow service members (whom he did not specifically name) in war crimes charges being investigated by Congress.
Split into simpler sentences: Kerry "had" served in Vietnam, but became better known after his discharge. Kerry made speeches and testimony opposing the war and implicating fellow service members (whom he did not specifically name) in war crimes charges being investigated by Congress.