The Streibs once invaded Minbari territory, but it proved a terrible mistake, as the superior Minbari fleet pursued them all the way to their homeworld and "made sure they understood the depth of their mistake" (in the words of Delenn).
The Streibs look (and behave) a bit like the Greys of UFO folklore, although in the Babylon 5 universe, what we call Greys are in fact the Vrees.