ruler's son was not old enough to take command, power passed to the late ruler's brother. This worked in the first generation but could lead to civil war in the second generation. The first time this happened, in 60 BC, the weaker party adopted what Barfield calls the 'inner frontier strategy.' They moved south and submitted to the dominant Central Plain regime and then used the resources obtained from their overlord to defeat the Northern Xiongnu and re-establish the empire. The second time this happened, about 47 AD, the strategy failed. The southern ruler was unable to defeat the northern ruler and the Xiongnu remained divided. Ethnolinguistic origins There are several theories on the ethnolinguistic identity of the Xiongnu. Proposed link to the Huns The Xiongnu-Hun hypothesis was originally proposed by the 18th-century French historian Joseph de Guignes, who noticed that ancient Chinese scholars had referred to members of tribes which were associated with the Xiongnu by names which were similar to the name "Hun", albeit with varying Chinese characters. Étienne de la Vaissière has shown that, in the Sogdian script used in the so-called "Sogdian Ancient Letters", both the Xiongnu and the Huns were referred to as the γwn (xwn), which indicates that the two names were synonymous. Although the theory that the Xiongnu were the precursors of the Huns as they were later known in Europe is now accepted by many scholars, it has yet to become a consensus view. The identification with the Huns may either be incorrect or it may be an oversimplification (as would appear to be the case with a proto-Mongol people, the Rouran, who have sometimes been linked to the Avars of Central Europe). Iranian theories There is a general consensus among scholars that the Xiongnu elite were originally Iranian. Harold Walter Bailey proposed an Iranian origin of the Xiongnu, recognizing all of the earliest Xiongnu names of the 2nd century BC as being of the Iranian type. Central Asian scholar Christopher I. Beckwith notes that the Xiongnu name could be a cognate of Scythian, Saka and Sogdia, corresponding to a name for Northern Iranians. According to Beckwith the Xiongnu could have contained a leading Iranian component when they started out, but more likely they had earlier been subjects of an Iranian people and learned the Iranian nomadic model from them. In the 1994 UNESCO-published History of Civilizations of Central Asia, its editor