The development of the JRV was further helped in late 1945 with the creation of the Aeronautical Union of Yugoslavia (Vazduhoplovnni Savez Jugoslavije - VSJ).Virtually all of the initial equipment was supplied by the Soviet Union – the aircraft captured during the war had quickly been retired.By the end of 1947, the JRV had reached a strength of some 40 squadrons of aircraft and had become the most powerful air arm in the Balkans.The country was immediately subjected to extreme political pressure from the Soviet Union and its Balkan neighbours, and the JRV's previous sources of aircraft, spares and fuel were cut off.However, the first-line strength of the JRV was still declining, so in 1951 the Yugoslav Chief of Staff, Colonel-General Koča Popović, visited the United Kingdom to discuss the situation.At the same time, a number of Yugoslav pilots underwent jet flying training in Germany and Colorado, USA.Ten Westland Dragonfly helicopters were obtained in 1954, and in 1956, after numerous delays due to political considerations, 121 F-86E/Canadair CL-13 F.4 Sabres interceptors were delivered.The Galeb was a big success, and it was better than Aero L-29 Delfín[verification needed]which was the main trainer aircraft in the Warsaw Pact and a number of another air forces.At the end of 60's JRViPVO purchased a number of Soviet MiG-21's in MiG-21PFM fighter, MiG-21R reconnaissance and MiG-21U and US trainer versions, fifteen Polish Mi-2 light helicopters, twenty five Zlin Z.526M Trainer Masters for Aviation Military Academy at Zemunik airport and the delivery of Mi-8T medium transport helicopters had started.The biggest project, Soko J-22 Orao attack aircraft made in cooperation with Romanian IAR, also started.JRV created its display team, Leteće Zvezde (Flying Stars) with seven Soko J-21 Jastreb aircraft which were later replaced with seven Super Galebs.Each Corps of AF and AD had one Fighter Aviation Regiment (Lovački Aviacijski Puk - LAP).Željava was one of the best airbases in Europe, with underground hangars, facilities for technical support, most advanced radars and communication equipment, sources of electricity, drinking water, fresh air, food, equipment, arms and fuel for staying 30 days without any connections with outside world.The main base was in Zadar, in 105th Fighter-Bomber Aviation Regiment, where young pilots made their first flights on Utva 75 aircraft.After finishing studies, cadets of Air Force Military Academy "Maršal Tito" become pilots and joined their service units.There was also one mixed-aviation squadron, 333.AE from 105.LBAP which had few Gazelle helicopters used for training by the Marshal Tito Air Force Military Academy.In 1991 the deep rooted grievances that had been threatening the unity of the Federal state for some time finally came to a head when Slovenia initiated moves towards independence.The JRV flew several low passes in a show of force against Croatia and launched a number of strikes.In August 1991 Serbian dominated federal government began an open war campaign against the Croats.Prior to this time, the Air Force had been mainly equipped combat aircraft of US origin, such as the F-84G and T-33A, and the US designation was commonly used.The main designation consisted of a prefix letter signifying the principal role of the aircraft, and a two-digit individual type number, e.g.: J-22.