Was Pink Diamond’s rebellion really the first one?

I've always wondered about Yellow Diamond's army and all the Gem Wars that get mentioned throughout the show. In Yellow's song, she talks about how “an army has a use — they can go and fight a war.” So what kind of wars are actually happening? Are there other Gems or even other Diamonds out there, maybe in other galaxies? Raising such a massive army just to deal with a few unruly Gems feels like overkill. Especially since, as we see in the show, the rebellion didn't really become a serious threat until Pink Diamond became Rose Quartz. So what was the army really meant for all this time?