This lively plaque is a fine example of the Greek style in Scythian art of the 4th century B.C. This tradition merged Greek artisanship with the most realistic depictions of Scythian daily life.
The battle scene depicted shows two vignettes which exemplify the costume and weaponry of the period. The scene to the left shows two Scythian warriors similarly clad in Greek-inspired armor. See, for example, the modern reconstruction, no. 10, p. 112-113 in Reeder, ed., Scythian Gold, complete with shirt and leg coverings formed of iron plates over leather, the leather bow case (gorytus) and scabbarded short sword. On the Romanovich-Stoclet plaque, the figure to the left carries a short sword, and the fallen warrior holds his bow.
The more gruesome scene to the right shows the decapitation of the enemy by the victorious Scythian warrior. Here clad in the customary Scythian leather coat and wearing a long shaggy, pointed beard, this warrior finds a parallel in depictions of Scythians on a gold vessel, now in the Hermitage, no. 146 in Aruz, et al., eds., The Golden Deer of Eurasia. The two costume types are found side-by-side on the famous comb from the Solokha kurgan, no. 156, in Aruz, op. cit., where the mounted soldier dons the armor, and one foot soldier wears the long leather coat and carries the typical weaponry. For the gorytus hung on a tree, as above the decapitated victim, see no. 212, op. cit.
The iconography of the beheading has been studied extensively by Knauer, op. cit. This scene perfectly illustrates the Scythian practice of presenting the head of the fallen enemy to one's leader, as described in Herodotus, IV, 64ff., "The heads of all enemies killed in battle are taken to the king; if he brings a head, a soldier is admitted to his share of the loot; no head, no loot."
For a battle scene on a helmet with similar Scythian costume, depicting the moment just before decapitation, see no. 124 in Reeder, op. cit.