Introduction
In the 1920s, at the beginning of the ‘age of aviation’, the Jesuit French priest Father Antoine Poidebard undertook one of the world's first aerial archaeological surveys, using a biplane and a camera to document hundreds of ancient forts and other sites throughout what today is Syria, Iraq and Jordan (Poidebard Reference Poidebard1934). Having piloted a biplane during the First World War, Poidebard later became a priest at Université Saint-Joseph in Beirut and joined the 39th Aviation Regiment of the French Levant forces, through which he began his expansive aerial survey of desert regions. Although today Poidebard is remembered primarily for his technological innovation in using aerial photography as an archaeological survey tool (Griswold Reference Griswold2019), an achievement that also fascinated his contemporaries (e.g. Dussaud Reference Dussaud1934; Jones Reference Jones1934; Hopkins Reference Hopkins1935), the substance of his investigation was based on mapping Roman-period forts and defensive installations along the eastern periphery of the empire. In his magisterial monograph, La Trace de Rome dans le desert de Syrie (Reference Poidebard1934), Poidebard presents hundreds of previously unknown forts and other sites over an area that stretches more than 1000km along the Roman frontier or limes (Figure 1).
Figure 1. A sample of Poidebard's (Reference Poidebard1934) aerial photographs: A) fort at Qreiye; B) Roman fort and medieval caravanserai at Birke; C) fort at Tell Zenbil; and D) castellum at Tell Brak.
Since the 1930s, historians and archaeologists have debated the strategic or political purpose of this system of fortifications (e.g. Graf Reference Graf1978; Liebeschuetz Reference Liebeschuetz and de Blois2007; de Jong & Palermo Reference De Jong, Palermo, Düring and Stek2018), but few scholars have questioned Poidebard's basic observation that there was a line of forts defining the eastern Roman frontier. This article presents the results of a regional-scale remote sensing-based survey utilising declassified CORONA and HEXAGON spy satellite imagery from the 1960s and 1970s (Casana Reference Casana2020a & Reference Casana, Lawrence, Altaweel and Philipb), which demonstrate there are far more forts than previously recognised, extending over a much larger region. As part of a systematic programme to map archaeological sites and features across a study area in the northern Fertile Crescent, stretching from western Syria to north-western Iraq, we found 396 previously undocumented forts or fort-like buildings, compared with the 116 forts recorded by Poidebard (Reference Poidebard1934) in the same area. Significantly, the spatial distribution of the forts we have mapped no longer supports Poidebard's central thesis that they constituted a broadly north-south line along the eastern boundary of the empire. Instead, we show that the forts form a roughly east-west line following the margins of the inland desert, connecting Mosul on the Tigris River in the east with Aleppo in western Syria. While many questions remain, our results fundamentally challenge understandings of the number, distribution and function of these distinctive elements of the ancient Near Eastern landscape.
Background
In his aerial survey, Poidebard (Reference Poidebard1934) set out to search for evidence of Roman-period fortifications along the eastern frontier of the empire. He focused on a route stretching from Borsa on the Jordanian border, north through Palmyra and Raqqa, south-east along the Euphrates River to its confluence with the Khabur, then north again to Nisibis on the modern-day Turkish border, ending at the Tigris River (Figure 2). During the reigns of Septimius Severus (AD 145–211) and Diocletian (AD 284–305), the Roman Empire made massive investments in military and transport infrastructure along its eastern border (Oates Reference Oates1968), and Poidebard's survey followed a road built under Diocletian, the strata Diocletiana. Poidebard recorded hundreds of fortified military buildings, including some of the best-known Roman forts in the region (Figure 3). He photographed all of these forts from the air and investigated many on the ground as well, conducting soundings at several of them and larger-scale excavations at the castellum at Tell Brak, Syria (Figure 4). Based on this evidence, Poidebard (Reference Poidebard1934) argued that the forts were mostly constructed during the second and third centuries AD, essentially functioning as a wall to fortify the eastern Roman provinces against incursions by Arab nomads or Persian armies.
Figure 2. Poidebard's (Reference Poidebard1934) map of the Roman limes, including the hypothesised route of principal road (in bold) that Poidebard followed on his aerial survey.