In more modern times, they were used by both the Allies and Axis during World War II to puncture the tires of wheeled vehicles. Improvised caltrops, usually made of welded nails, have also been used occasionally by labor activists in the United States during strikes to destroy the tires of vehicles belonging to management and replacement workers. In the labor context, these devices are more often called jack rocks. A recent example of the use of jack rocks in labor disputes was in 2012, allegedly by two men on a picket line at an aluminum products plant in West Virginia.