Not much is really known about 64 Wars. It was developed by Hudson Soft and planned to release on January 29th 1999, but was delayed to late 1999 and never released. It was supposed to release on the Nintendo 64 and could connect to Game Boy Wars 2. The game was supposedly shown at Space World 1999, there's a magazine article about it, and has a promotional gameplay video on VHS that's been uploaded to YouTube, but there really isn't too much known outside of that. Or probably better described, there's not much more that I know about it. Though even the Wars wiki page for it is pretty barren...
That being said, back in 2022 I was linked by AskAdvanceWars a flyer about the game that someone posted on Twitter. I, stupidly, didn't save it. Said Twitter user deleted their account. I can't seem to find another scan of it anywhere, so I guess keep an eye out for that if you want to look into this game more.