Although the Roman sandal may be the most iconic footwear from Antiquity, it was certainly not the only type worn within the Roman Empire. Sandals, such as the sandalium, were primarily intended for indoor use or in the warm Mediterranean region. They were light, open, and unsuitable for the heavy strain of long military marches.

In contrast, there were the caligae: sturdy, open military boots worn exclusively by soldiers and comparable to modern combat boots. The caligae were used by both legionaries and auxiliary troops, including the cavalry. Due to their close association with soldier life, Roman military personnel were sometimes referred to as caligati (“those who are shod” or “boot wearers”).

The caligae were designed with the harsh conditions faced by Roman legionaries in mind. Soldiers sometimes had to march tens of kilometers a day over rough terrain, which was extremely taxing on footwear. Instead of thick leather, wooden, or cork soles, the Romans used shoe nails: small nails with a head, driven through the sole, providing both grip and durability. 

The hobnailed sole of the caligae also indirectly offered a weapon function: a kick with such a shoe must have caused serious injury. Equally impressive was the thundering sound of an advancing army of caligati with their hobnailed-shoes. According to the Tannaim (Jewish scholars in Roman Judea), this sound once caused panic among Jews who were hiding, resulting in a deadly stampede (Babylonian Talmud, Shabbat 60a). Following this incident, the use of caligae within the Jewish community was restricted.

Thanks to the open design, the caligae ventilated well, helping to prevent blisters and allowing wet feet to dry faster. This was a great advantage, especially in warm climates. In colder regions, such as Northern Europe, socks were used to provide extra protection. However, wet wool in open caligae could be problematic, eventually leading to the development of more enclosed footwear.

Although socks are often associated with colder climates, they have been archaeologically found as far back as Egypt. Socks provide protection against friction from the leather and against cold. In northern forts such as Vindolanda (near Hadrian's s Wall in present-day England), evidence has been found that soldiers often had two types of shoes: open caligae and lighter closed shoes. Closed shoes offered better protection against the often wet climate of central and northern Europe. From as early as the 1st century AD, the Romans began to wear closed footwear in addition to the well-known open shoes. 

The consumption of caligae in the Roman army was enormous. A single cohort of 500 soldiers was estimated to use about 3,000 pairs of shoes per year, which amounts to medium three pairs per soldier every four months. In Vindolanda, where approximately 1,500 soldiers were stationed, more than 7,000 shoes have been found—a testament to the scale at which the Roman army wore out shoes.

Given the enormous consumption, it is plausible that within military forts, specialized shoemakers were active, fully dedicated to the production and maintenance of caligae. In a military context, this was a full-time activity. The existence of clear regulations is evident from an edict by Emperor Augustus in 31 BC, which stipulated how much shoe nails a pair of caligae should contain. This kind of standardization ensured quality and uniformity within the legions.

The midsole and open upper of the caligae consisted of a single piece of high-quality cow or ox leather. To this leather, an outsole was attached with hobnails made of iron, sometimes bronze. The nail ends were hammered inward and covered with an insole. Like all Roman footwear, the caliga did not have a heel or elevated sole.

The sandals were laced over the middle of the foot and around the ankle. The Spanish scholar Isidore of Seville believed that the word caliga derived from callus (hard leather) or from the verb ligare (to bind). The style of the leather straps could vary by maker and region. However, the placement of the nails was fairly consistent and served for optimal grip and support, comparable to modern sports shoes. At least one provincial producer of military caligae is known by name.

A well-known example of the strict discipline within the Roman army concerns an incident under Emperor Vespasian (69–79 AD). When a military unit complained about the rapid wear of their footwear and requested compensation for new caligae, Vespasian responded with remarkable severity. Instead of granting their request, he ordered that the soldiers march barefoot from Puteoli (near Naples) to Rome. This punishment emphasized the imperial disdain for complaints about equipment and demonstrated how limited the space for protest within the army was. The incident also illustrates how Roman authority subordinated practical concerns of soldiers to discipline and obedience.

In the early Roman Republic, soldiers were expected to finance their own equipment, including their shoes. From the late republic, the state organized recognized suppliers within military contexts to ensure quality control. From the second century AD, soldiers received, in addition to their salary, a specific compensation for footwear: the calcearium, named after calceus, the Latin for shoe. This allowed soldiers to purchase their shoes from suppliers of their choice, within or outside the army.

Due to this decentralization of production, the civilian shoemakers' market began to blend with that of military suppliers. This led to technological and aesthetic innovations in Roman footwear. Local peoples adopted the design of the caliga or adapted it to their own style. Around this period, the caligae gradually fell out of use and were replaced by more enclosed shoes suitable for the northern climate and civilian context.

In De Bello Judaico (The Jewish War), Book 6.2.1, the Jewish historian Flavius Josephus describes a remarkable incident where a Roman centurion slipped on the slick marble floor of the temple in Jerusalem. The cause was his caligae with studded soles. Although caligae provide perfect grip in the muddy open field, they are slippery on stone or marble floors. The incident underscores the functional but also context-sensitive nature of Roman military footwear.

The Roman emperor Caligula (37-41 AD) received his famous nickname as a child. During military campaigns, he accompanied his parents, General Germanicus and Agrippina the Elder. His mother dressed him in a miniature uniform of a Roman centurion, complete with small soldier sandals, known as caligae. The soldiers were so charmed by the boy in soldier's attire that they affectionately called him Caligula, which literally means "little soldier sandal." Although the nickname was given with affection, Caligula would come to strongly despise it as an adult.