Anabasis: Scythed Chariots

In the great battle just described, there was an ineffective use of a rather innovative weapons system that was specifically designed to contest heavy infantry hoplite units. This was the scythed chariot, which is exactly what you would imagine it to be from the name. There's also a Wikipedia article on the subject.

For the most part these things were not highly effective. One can imagine them having a significant psychological effect on the unit being charged by them. 

Xenophon does tell of a time (395 BC), however, when several hundred Greeks, caught in the open by the Persians, were charged by just two scythed chariots, scattering the men and allowing many to be cut down by the cavalry (Hellenica, IV.1.17-19). Indeed, this was their proper function: to panic and disrupt the enemy, allowing mounted troops and infantry to charge the broken line.

Most famously, scythed chariots were used by Darius III against Alexander the Great at the Battle of Gaugamela in 331 BC. There were two-hundred such chariots, says Diodorus Siculus, designed to astonish and terrify the enemy.

"From each of these there projected out beyond the trace horses scythes three spans long, attached to the yoke, and presenting their cutting edges to the front. At the axle housings there were two more scythes pointing straight out with their cutting edges turned to the front like the others, but longer and broader. Curved blades were fitted to the ends of these" (Library of History, XVII.53.2; also Arrian, III.8).

Diodorus records that, when the chariots attacked the phalanx, the Macedonians beat their shields with their spears, creating such a din that the horses shied, turning the chariots back on the Persians. Those that continued forward were allowed to pass as the soldiers opened wide gaps in the line. Some horses were killed as they charged ahead but the momentum of others allowed them to ride through, the blades of the chariots severing "the arms of many, shields and all, and in no small number of cases they cut through necks and sent heads tumbling to the ground with the eyes still open and the expression of the countenance unchanged, and in other cases they sliced through ribs with mortal gashes and inflicted a quick death" (XVII.58.2-5).

They're the kind of thing Hollywood would love, but definitely not the tank of the ancient world -- and ultimately no real threat to the dominance of heavy infantry on the battlefield of the era.