As soldiers, the Border Reivers were considered among the finest light cavalry in all of Europe. After meeting one Reiver (the Bold Buccleugh), Queen Elizabeth I is quoted as having said that “with ten thousand such men, James (VI) could shake any throne in Europe.” Reivers served as mercenaries, or were forced to serve in English and Scots armies in the Low Countries and in Ireland. Such service was often handed down as a penalty in lieu of that of death upon their families.
Reivers fighting as levied soldiers played important parts at the battles of Flodden Field and Solway Moss. When fighting as part of larger English or Scottish armies, Borderers were difficult to control as many had relatives on both sides of the border, despite laws forbidding cross-border marriage. They could claim to be of either nationality, describing themselves as Scottish if forced, English at will and a Reiver by grace of blood. They were badly-behaved in camp, frequently plundered for their own benefit instead of obeying orders, and there were always questions about how loyal they were.
At battles such as Ancrum Moor in Scotland in 1545, borderers changed sides in mid-battle, to curry favour with the likely victors, and at the Battle of Pinkie Cleugh in 1547, an observer (William Patten) noticed that the Scottish and English borderers were talking to each other in the midst of battle, and on being spotted put on a show of fighting.