The Battle of the Oxus occurred in 399, when Chandragupta II defeated the Hunas at the banks of the Oxus river.
Background
Submission of Varahran
After the (Persian) Sasanians faced defeat in the Battle of Sistan, which demoralized the Persian contingents in present-day Afghanistan. As the Gupta Army marched northwards to Kapisa, Varahran was quick to grasp the political realities and offered his submission to the Gupta Emperor, Chandragupta II.
Battle
Gupta cavalry's arrival by the Oxus river
Bactria was under the Huna occupation in the last quarter of the fourth century AD. The sudden attack into the Oxus valley caught the Transoxiana alliance off-guard. The Pamir Tocharians were unable to combine with the Hunas (Hephthalites). On hearing the news of the Gupta Empire advance, the Hephtalites resorted to a tactical retreat to the north of the Oxus River into the plains of Southern Uzbekistan. When the Gupta cavalry arrived by the Oxus river on the southern banks, they camped there. Kalidasa poetically described how the cavalry camped on the banks of the river Vankshu in the midst of saffron fields in a verse of his Raghuvamsa:
"...His horses, that had lessened their fatigues of the road by turning from side to side on the banks of the river Vankshu (Oxus), shook their shoulders to which were clung the filaments of saffron..."
Historians studied this as a description of the Gupta cavalry camping on the banks of the Oxus during Chandragupta II's expedition.