I cannot remember if this one is a novel, or a series, or an anthology. I can't even think of who the author is, except I'm confident that it's not Heinlein.
Humans have just discovered FTL and are conquering the space around their system. The story is from the point of view of a General or a politician, who is from an alien empire. Just as they decide to do something about it they receive a notice from a world they have never been able to communicate with, and it's a simple "Don't."
It's been at least 35 years since I read this and it's entirely possible I'm just remembering a mix and match of stories.