In order to raise some US generals to equal rank with British Field Marshall Montgomery, the army added a fifth star arranged in a pentagon like shape. The extra star was presented to Generals Dwight D. Eisenhower and Omar Bradley along
with the rank General of the Army. Nimitz was the only Admiral to receive a fifth star! The portrait here was while he was still Fleet Admiral Nimitz! That practice along with the fifth star was discontinued after WWII!
There is an early precedent for that. During the American Civil War the highest rank in the US Navy was commodore (commander today) in order to put Naval officers on an equal footing with Maj.General Ulysess S. Grant the rank of Admiral was created. On the rivers Mississippi and Tenessee for the first time in US history, these were combined Army, Navy operations.