According to archaeologists of the former Soviet Union, a serious social crisis struck the Bactria region in the latter half of the 4th century, the so-called "early medieval period", which means a transitional phase from the ancient slave society to the medieval feudalism. This article examines this hypothesis. First, we check the archaeological data concerning Buddhist sites in Bactria. At Kara Tepa in Uzbekistan, we can date the life of the sites by investigating the pottery and coins. Those archaeological data indicate that a broad area was used for burial sites at the end of the 4th century. The same decline could be observed in the latter half of the 4th century at the second Buddhist temple of Dal'verjin Tepa, where we can safely date the end of the usage of the building as a Buddhist site by the radiocarbon dating and the analysis of the that several political powers such as Sasanians, Kushano-Sasanians, Alkhans and Kidaras entered into Bactria and Gandhara after the collapse of the Kushans. Presumable conflicts and struggles among those powers could have prevented regular connections, which had been maintained theretofore, between the north and the south of the Hindukush. This means that the influence of Gandharan Buddhism no longer reached Bactria through the traditional trade routes. Moreover, we can easily imagine that the rulers of Bactria at the end of the 4th century (presumably Sasanians judging from Bactrian documents) had no interest in Buddhism. In this way, Bactrian Buddhism lost its patrons and gradually declined because of political and economic problems. pottery excavated from the floor. Although the examples we have are not exhaustive, we do not find so far any evidence that demonstrates the continuity of other Buddhist temples in Bactria beyond the end of the 4th century. Second, we review ancient cities and forts in the region in question. Some archeologists of the former Soviet Union said that almost all cities had declined by the end of the 4th century and new, small castles or fort-like buildings appeared in the 5th century, which marked the beginning of medieval feudalism. However, from the archaeological point of view, there remain cultural layers of the 5th century at Dal'verjin Tepa and Zar Tepa. In addition, Kuyovkurgan and the citadel of Zar Tepa, both said to be typical fort-like buildings appearing in the middle of the 5th century, turn out to have been constructed in the 6th-7th centuries judging from the pottery assemblage. Thus, we can say that the collapse of old cities and the emergence of new fort-like buildings weren't concentrated in the latter half of the 4th and the 5th centuries. As seen above, the archaeological data suggest that it is difficult to suppose the existence of a "social crisis" in the whole Bactria region. Why then did only Buddhist sites temporarily decline in the latter half of the 4th century ? From recent studies, we know that several political powers such as Sasanians, Kushano-Sasanians, Alkhans and Kidaras entered into Bactria and Gandhara after the collapse of the Kushans. Presumable conflicts and struggles among those powers could have prevented regular connections, which had been maintained theretofore, between the north and the south of the Hindukush. This means that the influence of Gandharan Buddhism no longer reached Bactria through the traditional trade routes. Moreover, we can easily imagine that the rulers of Bactria at the end of the 4th century (presumably Sasanians judging from Bactrian documents) had no interest in Buddhism. In this way, Bactrian Buddhism lost its patrons and gradually declined because of political and economic problems.
