Although it was used for centuries by numerous armies, the unobtrusive-looking device called the caltrop, or calthrop, has often been overlooked by military historians... In many respects, the caltrop is the ideal passive weapon– simply constructed, cheap and easy to manufacture, requiring no special skill or training to use, easily portable, needing no care, maintenance or preparation, capable of recovery and, above all, extremely effective in most settings... And unlike other weapons, it has never been completely replaced by more modern descendants.

Robert W Reid, Weaponry: The Caltrop. Originally published in Military History magazine, 1998

Used by the Huong Kha army in Vietnam against the French troops, from 1885-1896. On display in the National Museum of Vietnamese History, Hanoi.

Gary Todd, Vietnam Under French Rule, 19th Century, Caltrops Used Against the French,