When you read old texts you can observe that certain people (who were kings) were also given this name which implied they were kings. You could for example find "Oedipus rex" which means Oediupus the king, this is also how we know (a way at least) what this meant.
Explanation: It was the firt major ground assualt against the japanese and it turned out to be a allied victory which put the japanese on the defensive allowing allied forces to go on the offensive
