Short answers, pulled from the story.
Romania officially joined the Axis powers on the 23rd of November 1940. This followed the September 1940 formation of the National Legionary State under Ion Antonescu in partnership with the Iron Guard.
In the summer of 1940 the USSR occupied Bessarabia and Northern Bukovina after an ultimatum. Under the Second Vienna Award, Hungary received Northern Transylvania, and under the Treaty of Craiova, Bulgaria received Southern Dobruja. Germany and Italy arbitrated these losses.
Romania had 686,258 men under arms in the summer of 1941, rising to 1,224,691 by the summer of 1944. The number of Romanian troops sent against the Soviet Union exceeded that of all of Germany's other allies combined.
Romanian troops were responsible for the persecution and massacre of Jews in Romanian-controlled territories, and a 2004 government commission found between 280,000 and 380,000 Jews murdered or died on Romanian soil. The report concluded that Romania committed genocide against the Jews and bears responsibility for more Jewish deaths than any Nazi ally other than Germany itself.
King Michael I led the coup on the 23rd of August 1944, deposing the Antonescu dictatorship. He proclaimed loyalty to the Allies and declared war on Germany, with support from opposition politicians, most of the army, and Communist-led civilians.
Despite switching sides, Greater Romania was not restored except for Northern Transylvania, which was recognized as an integral part of Romania. Under the 1947 Treaty of Paris, Romania was still labeled an "ally of Hitlerite Germany" and had to pay $300 million in reparations to the Soviet Union.