Sightings of enemy vehicles during the month of May totaled 54,561--seven times those of January. This increase prompted General Van Fleet to ask the Fifth Air Force and Seventh Fleet to initiate a program of cutting off all possible enemy road traffic between the latitudes 38° 15´ N and 39° 15´ N. Earlier in 1951 the interdiction program had been aimed chiefly at the enemy’s rail lines and bridges. The Communists had countered by using more trucks. The new program, known as Operation STRANGLE, was to be concentrated against vital road networks. Flight leaders were briefed to search out critical spots where truck and ox cart traffic could be stopped. Roads skirting hills were to be blocked by landslides caused by well placed bombs. Where cliffside roads followed the coast, as they so often did in East Korea, naval gunfire started avalanches of dirt and rocks which sometimes reached a depth of 20 feet. Roads running through a narrow ravine or rice paddy could often be cut by a deep bomb crater.[226] [226] Descriptions of Operation STRANGLE are based on _Pac Flt Interim Rpt_ No. 3, Chapter 10, 10-45 to 10-47; and on 1st MAW _HD_s, May to Jul 51, G-3 PORs, G-3 Journal entries, Assessment Rpts. The 1st MAW was given the assignment of stopping traffic on three roads in East Korea--from Wonsan to Pyonggang, from Kojo to Kumhwa, and along a lateral route linking the two (Map 16). Since Kumhwa and Pyonggang were two of the three Iron Triangle towns, these roads were of more than ordinary importance. The Communists reacted to the new UN pressure by increasing their flak traps. UN pilots were lured with such bait as mysterious lights, tempting displays of supposed fuel drums, or damaged UN aircraft that called for investigation. The cost of the UN in planes and pilots showed an increase during the first two months of Operation STRANGLE. From 20 May to the middle of July, 20 Marine planes were shot down. Six of the pilots returned safely; two were killed and 12 listed as missing.[227] [227] 1st MAW _HD_s May-Jul 51, Summaries; MAG-12 and MAG-33 _HD_s May-Jul 51, Summaries. The demands of Operation STRANGLE added to the emphasis on interdiction and armed reconnaissance by the Fifth Air Force. Statistics compiled by the 1st Marine Division for 1-17 June 1951 show that 984 close air support sorties had been requested and 642 received--about 65 percent. The ratio of Marine planes to other UN aircraft reporting to the Division was about four to one.[228] [228] Summarization from DivAirO memo of 26 Jun 51 to CG 1stMarDiv.