On 23 August 1944, with the Red Army penetrating German defenses during the Jassy -- Kishinev Offensive, King Michael I of Romania led a successful coup against the Axis with support from opposition politicians and most of the army. Michael I, who was initially considered to be not much more than a figurehead, was able to successfully depose the Antonescu dictatorship. The King then offered a non-confrontational retreat to German ambassador Manfred von Killinger. But the Germans considered the coup "reversible '' and attempted to turn the situation around by military force. The Romanian First, Second (forming), and what little was left of the Third and the Fourth Armies (one corps) were under orders from the King to defend Romania against any German attacks. King Michael offered to put the Romanian Army, which at that point had a strength of nearly 1,000,000 men, on the side of the Allies. Surprisingly, with the Red Army occupying parts of Romania, Stalin immediately recognized the king and the restoration of the conservative Romanian monarchy. (Deutscher, Stalin. 1967, p. 519)
when did romania join the allies in ww2
23 August 1944