Strabo (XI.vii.2) tells us that the tribes of “Asii, Pasiani, Tochari, and Sacarauli” were the nomads “who originally came from the country on the other side of the Jaxartes river that borders that of the Sacae and the Sogdiani and was previously occupied by the Sacae.” This is the attack in which these groups took the control of Bactria from the Greco-Bactrians, most likely during the time of Eucratides I.
Among these tribes, the Asii (or Asiani) seem to have taken over and become the king of the Tocharii, and the Saraucae (Strabo’s Sacarauli?) to have been destroyed, as Pompeius Trogus tells us (Prologue, XLI-XLII).
This matches remarkably well with the description of the rise of the Kushans (Guishuang) from among the five Yabghus of the Yueh-zhi as described by Chinese sources (Hou Han Shu 118.9a). The problem seems to be that the description of the Tochari in this narrative, and their submission to the Asii does not fit very well with the status of the Da Yueh-zhi is given by the Chinese sources (Errington and Curtis, 104, note 59). The Da Yueh Zhi are too big to be just another tribe like the Asii, Pasiani, and the Sacarauli!
I think it would not to be too emphasised that this is the question of a point of view. The Yueh-zhi, or what was recognised by the Chinese as the Da Yueh-zhi were an important menace for the Chinese on their western door steps. But the importance attached to them by the Chinese need not be necessarily an absolute case, making them larger or more important than other tribes. In fact, that they are put at an equal footing with the Sacarauli, presumably a Saka confederacy, might show this supposed problem of “lack of distinction” in the case of the Tochari (presumably the Yueh-zhi).
We know that Phraates II (139/8-127 BC) was killed in battle against the Sakas, as was his uncle Artabanus I (127/126-122). Coins of Phraates II found in Giaour Kala in the Marv Oasis are struck over issues of Eucratides I, the king of Bactria during the attacks reported by Strabo. This synchronisation would mean that the Sakas who came to kill Phraates II were part of the same group that had earlier taken Bactria from Eucratides (see Errington and Curtis, 55). If we then trust the report of Strabo that these Sakas (Sacarauli?) had moved over Jaxartes together with the Asii, Pasiani, and the Tochari, this would show that in their initial phase of action, the Tochari and the Sacarauli, namely the Yueh-zhi and the Sakas, were on equal footing. The latter, the Sacarauli/Sakas even occupied a more important place in the affairs of the west than the Yueh-zhi/Tochari, who were more important for the Chinese sources. This would also provide a terminus post quem of 139 (the beginning of the rule of Phraates II) for the attack of these tribes on the Parthian lands and a date close to that for their attacks on Bactria.
It would not be absolutely insensible to suggest that with the attacks of the Sakas on Bactria, Phraates II took advantage of the weakening Bactrian situation to take over Satrapis of Turiva and Aspianos (Strabo XI.xi.2), probably the Marv Plain, from Eucratides. This brought him into direct conflict with the Sakas, possibly located farthest west among the four tribes. The Sakas thus defeated and killed Phraates, and then his uncle Artabanus, before moving south into Areia, then Arachosia and Drangiana. There, they were defeated by Mithridates II and a branch of them was settled in Drangiana/Arachosia (the future Sakastan/Sistan), while an eastern branch headed by Maues founded the Indo-Scythian kingdom in Gandhara.
Meanwhile, the Tochari/Yueh-zhi had remained in northern Bactria/Eastern Sogdiana, where the Asii/Kushan managed to gain ascendancy over them. This goes well with the report of the Hu Han Shu on the rise of the Guishuang. It has been previously argued that the “Five Yabghus of the Yueh-zhi” is not an internal division of the Yueh-zhi themselves, but an administrative setting of Bactria adopted by the nomadic tribes. In this case, we can imagine that one of these divisions was controlled by Strabo’s Asii, the Gueshuang of the Hu Han Shu, who came to control the other four, occupied by the Tocharo/Da Yueh-Zhi, and probably the enigmatic Pasiani. The Asii/Gueshang/Kushans, at the head of their new confederacy, then followed the example of the Sakas and started moving south, crossing the Hindu-Kush, first arriving in the Kabul Valley/Kapisa where they competed with Maues’ successor Azes I and the with the “Indo-Parthian” king of Sakistan, Gondophares, as can be demonstrated from the overstrikes of these authorities over each others coins in the Kapisa region (Senior I).