Known Space is divided into Spheres, which are autonomous governmental groupings of one or more settled star-systems. They differ widely in their wealth, style of government, power and size. Humanity has also encountered several sentient extraterrestrial species in its travels. Their psychologies have been quite alien to humans, though a couple of these races (the Hammerheads and the Starwhales) have proven to be useful allies to humanity. One of these races, though, the Tripods, was initially so hostile that their contact brought an almost immediate war that threatened the majority of humanity. As of 2221, the Tripod War has only been out of the picture for about six years.