Short answers, pulled from the story.
The 1944 Romanian coup d'état, known in Romanian historiography as the Act of the 23rd of August, was a coup led by King Michael I on the 23rd of August 1944. It removed Prime Minister Ion Antonescu's government, which had aligned Romania with Nazi Germany, and brought Romania into the Allied camp. Historian John Lukacs called it the most successful coup of World War II.
King Michael I led the coup, supported by a coalition called the National Democratic Bloc, which included the Romanian Communist Party, the Social Democratic Party, the National Liberal Party, and the National Peasants' Party. On the communist side, Emil Bodnăraș and Lucrețiu Pătrășcanu were the primary conspirators, according to Silviu Brucan.
The first meeting between King Michael's representatives and the communists took place on the night of the 13th to the 14th of June 1944 at a communist safe house at 103 Calea Moșilor in Bucharest. Military representatives, royal advisers, and communist organizers all attended.
Ion Antonescu was arrested in the royal palace on the 23rd of August 1944. The new prime minister, Lt. Gen. Constantin Sănătescu, handed custody of Antonescu to Romanian communists, who turned him over to the Soviets on the 1st of September. He was later returned to Romania, tried, and executed in 1946.
King Michael I received the Soviet Order of Victory from Joseph Stalin in 1945 and the highest degree of the Legion of Merit, Chief Commander, from President Harry S. Truman the following year. Despite these honors, he was forced to abdicate in 1947 and lived in exile until 1992.
Albert Speer considered Romania's defection economically decisive because it deprived the Axis of Romanian oil and cut off access to Turkish chromium. Within a month of the coup, Bulgaria and Finland also changed sides, the Slovak National Uprising began, and a failed coup was attempted in Croatia on the 18th of September 1944. The loss of Romanian oil prompted Hitler's first admission that the war was lost.