Special forces, or special operations forces are military units trained to perform unconventional missions. Special forces, as they would now be recognised, emerged in the early 20th century, with a significant growth in the field during the Second World War.Depending on the country, special forces may perform some of the following specializations: airborne operations, counter-insurgency, "counter-terrorism" , covert ops, direct action, hostage rescue, high-value targets/manhunting, intelligence operations, mobility operations, and unconventional warfare. The term Special Forces in the US refers to the US Army's forces while the term Special Operation Forces refers to all units. In Russian speaking countries special forces are typically called Spetsnaz, an acronym for "special purpose." Likewise Russian speakers refer to special forces of other nations as Spetsnaz, for example US Special Forces would be referred to as US Spetsnaz in Russian.