Exploratores and speculatores were divisions of the Roman army whose main purpose was to gather information for the legions about the enemy army and field conditions. The scope of their duties varied, but often the functions overlapped.
Both units were of great importance to the Roman Legion, whether in terms of choosing a battle strategy or ensuring the security of the military during the march.
Exploratores were scouts in the Roman army who were responsible for reconnaissance at long distances. Patrols, largely composed of horsemen, scoured the area to detect the enemy army and gather the necessary information about its location, activity and numbers. In addition, exploratores familiarized themselves with the terrain conditions, its weak and strong points, and designated a favourable site for the construction of the camp. Often a detachment of scouts was ahead of the regular army by a day’s drive.
Exploratores also obtained information from prisoners or deserters. Usually, the unit consisted of local warriors who served in the auxiliary troops. We know little about the exploratores organization, but they were probably subordinate to the centurion. The reconnaissance detachment was probably small in order to be able to effectively conduct intelligence.
Tacitus, a Roman historian, mentions that in 69 CE, the commander of Vespasian – Antony Primus – sent 4,000 riders towards Cremona to support the auxilia troops looting the area. During this time, exploratores carried out reconnaissance over longer distances. It is believed that their intelligence activities may have taken place approximately 12 kilometres from the Antonius ride. In this way, the Roman commander had a broader picture of the area and was aware of whether there was any threat to his legions. The next day, exploratores discovered the upcoming Vitellius troops, giving Antony enough time to prepare for the confrontation.
Another example of exploratores intelligence activities was in 58 BCE when Caesar led the Gallic campaign. The Roman commander headed the army from Vesontio (now Besançon in France) to confront the Ariovist, who probably led the union of the Suebi tribes, and the Helvetians. After six days of strenuous march, the exploratores squad noticed about 24 milia passuum from here (about 35 km) of the enemy force.
The Speculatores were a branch of the Roman army that aimed to gather as much information as possible about the enemy, his troops and activities through covert operations.
Such a unit often used Caesar to support exploratores. Both speculatores and exploratoreswere collecting information about the enemy army, but only the former were considered more spies. Certainly, speculatores were less numerous than exploratores.
Ammianus Marcellinus, a Roman historian, reports that in 365 CE the usurper Procopius sent speculatores to Constantinople to spread hostile rumours about Valentinian I. that Procopius was elected emperor and reigned from September 28, 365 to May 27, 366 CE.
Another known use case of speculatores – in 359 CE, Ammianus himself and the commander Ursicinus caught a Roman deserter, originally from Gaul, who acted as speculatores for Persians. He carried out reconnaissance and reconnaissance in the Roman military zone and sent reports to the Persian command.
Speculatores were mostly prisoners of war or deserters. Often the material collected by speculatores and exploratores overlapped.