The Yugoslav order of battle prior to the invasion of Yugoslavia was made up of the various operational formations of the Royal Yugoslav Army, Royal Yugoslav Air Force and Royal Yugoslav Navy immediately prior to the World War II Axis invasion of that country (map pictured), which commenced on 6 April 1941. The Yugoslav Army included 33 incompletely mobilised divisions, and most of the heavy weapons and armoured vehicles available to it were obsolete, most formations were heavily reliant on animal-powered transport, and it possessed only 50 tanks that could engage front line German tanks on an equal basis. The Yugoslav Air Force consisted of four almost completely mobilised air brigades with more than 423 aircraft of Yugoslav, German, Italian, French, Czech and British design, including 107 modern fighter aircraft, and 100 modern medium bombers. The small navy was centred on a destroyer leader, three smaller destroyers, four obsolescent submarines and a gunboat.