Actually it is, you just didn't read enough.....
Yamamoto Isoroku, original name Takano Isoroku, (born April 4, 1884, Nagaoka, Japan—died April 18, 1943, Solomon Islands), Japanese naval officer who conceived of the surprise attack on the U.S. naval base at Pearl Harbor on Dec. 7, 1941.
Yamamoto graduated from the Japanese Naval Academy in 1904, and a year later he was wounded in action at the Battle of Tsushima during the Russo-Japanese War. In 1913 he enrolled in the Japanese Naval Staff College, and after graduating in 1916 he was adopted into the Yamamoto family and changed his name.
I read it all. Why would he be publicly and professionally known by his given name rather than family name?? Or is this some modern re-arrangement that reverses generations of nomenclature?