The first known officially sanctioned assassination by the United States of an enemy leader was when Roosevelt authorized[1] the Navy to intercept the airplane carrying IJN Admiral Isoruku Yamamoto[2] on an inspection tour of installations in the Solomon Islands. This was in April of 1943.

[1] Operation Vengenace

[2] The planner of the attack on Pearl Harbor, as well as the driving force behind the modern, carrier-centric Japanese Navy