Through the final years of the 1st century AD, the legions remained the backbone of the Roman army, although the "auxilia" in fact outnumbered them by up to half as much again. Within the legions, the proportion of troops recruited from within Italy fell gradually after 70 AD. By the close of the 1st century, this proportion had fallen to as low as 22 percent, with the remainder drawn from conquered provinces. Since technically only citizens were allowed to enlist in the legions, where recruits did not possess citizenship then, at least in some instances, citizenship "was simply given them on enlistment". During this time, the borders of the Empire had remained relatively fixed to the extent originally reached under the Emperor Trajan. Because of this, the army was increasingly responsible for protecting existing frontiers rather than expanding into foreign territory, the latter of which had characterised the army's earlier existence. As a result, legions became stationed in largely fixed locations. Although entire legions were occasionally transferred into theatres of war, they remained largely rooted in one or more legionary bases in a province, detaching into smaller bodies of troops (Latin: "vexillationes") on demand. This policy eventually led to a split of the military's land-based forces into mobile and fixed troops in the later Empire. In general, the best troops were dispatched as "vexillationes", and the remainder left to guard border defenses were of lower quality, perhaps those with injuries or near retirement.