Isn’t that a French expression on its own?
Yeah, like @mike said it’s from French. You’ll find that English borrows a lot of phrases from other languages, and from French in particular due to the Norman conquest.
Yes, it is French.
While it is used in English, I think it’s a bit weird to say it’s an English phrase in any way. By no means is it common parlance nor used in any formal way like de jure or de facto (from Latin).
For my fellow English-speakers… how many times have you used the phrase en garde in your life outside of childhood swordplay because some cartoon or movie taught it to you? My guess is never.