Soldiers from Latvia and Poland recently wrapped up the “Ridge Runner” training exercise in West Virginia which partnered them with American military forces to train in unconventional warfare. The training prepares these Eastern European soldiers for what was once known as the “stay-behind” mission, in the event Russia invaded their countries with conventional or irregular forces, as seen recently in East Ukraine.

The three main methods used by Special Operations soldiers to infiltrate their target areas is by land, air, or sea. That could potentially see these small teams utilizing parachutes, small boats, re-breather systems, driving off-road vehicles, or even disguising themselves and crossing an international border. However, the less understood method is the concept of stay-behind networks. These small cells are pre-positioned inside an area that may be invaded by the enemy. If the worst-case scenario happens, these cells activate and conduct acts of sabotage and espionage.

Their role would not be to defeat the enemy by themselves, but rather to degrade, delay, and frustrate the opposition in its rear areas by destroying rail junctions with explosives, cutting power lines, or ambushing enemy units. Today, these stay-behind cells may also act as forward observers, calling in NATO airstrikes from cellphones, or even using iPads like the Kurdish YPG has done in Syria.

“Ridge Runner’s mission is to develop and execute irregular warfare training across the state of West Virginia that contributes to the development of Special Operations Forces’ and General-Purpose Forces’ irregular warfare understanding and capabilities, in order to support national security,” according to a statement from an Army spokesperson.

The U.S. military ran a highly successful stay-behind unit called Detachment A in Berlin during the Cold War which was staffed by active duty Green Berets. However, the CIA and (allegedly) the 10th Special Forces Group were also involved in training civilian stay-behind cells all across Western Europe. Some of those cells belonged to the infamous Gladio network in Italy which conducted acts of domestic terrorism during a time so dark, it’s commonly known as Italy’s “Years of Lead.”