Russia mounted a gun on one of its space stations[0] but it didn't work out, and they were going to launch an orbital nuclear platform[1] but then everything went to shit.

Less flippantly, I seriously doubt the US is the first country to put weapons in space, they're just stupid enough to admit it for some reason. I would bet money I don't have that both Russia and China have something nasty up there at least, and the US has probably launched something for Israel at some point.

[0]https://en.wikipedia.org/wiki/Salyut_3

[1]https://nationalinterest.org/feature/the-rise-fall-the-sovie...