Caligae (the plural of 'Caliga') were the sandals in which the Roman army fought; they were the first footwear to be designed specifically for military use and are therefore the first ever
combat boots, although they are closer to modern jungle
hiking boots than the traditional hard black leather assault boot. Precise details of dates and construction are lost, but those that can be observed in paintings and preserved in bogs generally consisted of a sole held on the foot with interlocking leather straps. The sole was made of several layers of
tanned leather, roughly half an inch deep, studded with metal
hobnails. There were distinct left and right soles, something which sounds obvious today but was lost in the middle ages.
They were simple but served the Roman empire well; caligae were comfortable, hard-wearing, and above all extremely cheap to make. Then as now cost was a major factor; the soldiers - known as 'caligati' because the officers wore 'calceus', their own personal footwear - had to provide and insert their own hobnails, and indeed were called upon to repair and, if necessary, construct their own sandals. Times were harsher then. The hobnails were useful across country - and for trampling over fallen barbarians - but proved lethal on cobbled streets and over rock, in which case the caligae were often bound with leather for extra grip.
Being open-topped sandals, caligae were ideally suited for hot Mediterranean and North African environments, although they were less useful in Gaul during the winter months and the north of England, which is perhaps one reason why Scotland was never part of the Roman empire; sandals are not popular there nowadays, either. A more enclosed design and thick socks were used in the colder parts of the empire; and as the empire spread across the known world, the universal issue of caligae declined, such that by the end of the first century AD closed boots were more common. The gradual fall of the Roman Empire and its temporary erasure from history ensured that military footwear hardly progressed until the 1600s.
Whilst modern military boots are popular amongst the civilian population - especially with certain musical tribes - the caligae have yet to make a comeback, notwithstanding the efforts of New Model Army. Caligae are relatively easy to construct at home, and are available from military re-enactment suppliers.
As mentioned above, footsoldiers were known as 'caligati'. Emperor Gaius Julius Caesar Germanicus spent his childhood in military camps on the German frontier and had child-sized pairs, which were known as 'caligula', or 'little caligae'; it was under this name that he later became infamous.
For great justice:
http://www.larp.com/legioxx/caligae.html
http://www.albionarmorers.com/accoutrements/sandals.htm