South Waziristan is the southern part of Waziristan, a mountainous region of northwest Pakistan, that covers some 11,585 km² (4,473 mi²). Waziristan comprises the area west and southwest of Peshawar between the Tochi River to the north and the Gomal River to the south, forming part of Pakistan's Federally Administered Tribal Areas (FATA). The Khyber-Pakhtunkhwa lies immediately to the east. The region was an independent tribal territory from 1893, remaining outside of British-ruled empire and Afghanistan. Tribal raiding into British-ruled territory was a constant problem for the British, requiring frequent punitive expeditions between 1860 and 1945. Troops of the British Raj coined a name for this region "Hell's Door Knocker" in recognition of the fearsome reputation of the local fighters and inhospitable terrain. The region became part of Pakistan in 1947.