The Yuezhi were a (semi-)nomadic people or state on China's northwestern periphery, probably in what's now Gansu Province, until the mid-2nd century BC, when they were defeated by the Xiongnu*, and embarked on a great migration to finally end up on the northern bank of the Oxus (Amu Darya) in what's now southern Uzbekistan and Tajikistan. Their arrival there is apparently the earliest historical event to be mentioned in both Chinese and Greco-Roman histories. Eventually the Kushan empire would arise from them to encompass large chunks of Central Asia and India, but that's outside Benjamin's scope.

The archaeology and literary sources available don't allow Benjamin to tell this story in too much detail, so he pads out the (still rather modest at ~250pp) book by lengthy excursions on the pre-Yuezhi history of regions they crossed, and on the relations between Han China and the Xiongnu.

The introductory chapter on "Origin" spends a lot of space seeking to show that the Yuezhi spoke a language belonging to the Tocharian branch of Indo-European. The argument amounts to that various outsiders applied variants of the ethnic label "Tocharian" to both the Yuezhi and the undoubted Tocharian-speakers in the Tarim basin. Hardly conclusive, but more convincing than I expected given the dismissals of the idea I've seen elsewhere.

A somewhat frustrating book, partly because the sources allow Benjamin so few firm conclusions, partly because of various idiosyncrasies of his style and terminology, but ultimately quite interesting.

* Often called "Huns" in western works, but the connection, if any, to the western Huns of Attila is disputed.