Kings reign. Governments govern.

Only royalty, kings or queens, may "reign". Elected politicians, successful revolutionary leaders, or military dictators may "govern" (hold political power), but not "reign". The word "reign" comes form an old Indoeuropean root that has given us English "reign", Latin "rex", Sanskrit "Raja", Spanish "rey", French "roi", and German "Reich", among others. Although a few "kings" were elected or self-nominated, in general, reigning entails being the lawful and acknowledged inheritor of the title of "king" and having been crowned as such according to the laws of the country. In mediaeval times kings had to be acknowledged as such by the Pope and often by an assembly of the nobility. In many cases, too, kings have been considered representatives or even incarnations of deities, and many have explicitly pretended to "reign" by the will or the grace of God. Etc. ;-)