The mechanism of Roman line relief

[Duncan Head]

Exactly - a cavalry skirmish involving the Spanish sword, not some imaginary "sabre".

It's interesting that when the first real Republican-period gladii hispanienses were identified, like the Delos sword for example, some people suspected that they might be cavalry swords, because they were so much longer than expected. The Republican infantry Spanish sword was not a short stabbing sword at all, but a medium-length cut-and-thrust weapon - "It has an excellent point, and can deal a formidable blow with either edge" - as Polybios puts it.

[Mark G]

It needs to be a hell of a lot longer to be any use on horseback

[Duncan Head]

The gladius hispaniensis was about 65-75 cm long (the Delos sword is 76 cm overall). This is of the same order as the longest versions of the kopis, as used by Greek/Macedonian cavalry - one at http://comitatus.net/greekswords.html is 68.5 cm overall - so is presumably long enough for cavalry use.