As part of the Nazi-Soviet Pact of August 1939, a secret protocol was enacted that gave the Nazis the Balkans as their sphere of influence. Earlier in the year, Britain and France had tried to guarantee Romania’s borders, but Romania’s refusal to allow the Red Army to cross its borders kept Russia out of the pact, and the guarantee fell apart.
It's doubtful that the United Kingdom would have been in a position to do anything anyway as the Soviet Union and hungary forced concessions on the Romanians in 1940 to give away hundreds of square miles of territory containing 4 million people. Germany counciled the Romanians to accept the Hungarian demands.
This paved the way for the Iron Guard, the Romanian Nazi Party, to take power. General Ion Antonescu and rogue military officers forced the King to abdicate, leaving his young son, Michael, on the throne. They soon imposed a fascist regime. The Iron Guard butchered the King's helpless supporters in prison, and imposed anti-semitic laws; some half-million German troops entered the country. By war’s end some 300,000-400,000 Jews in Romania and Hungarian-held Transylvania were deported to the death camps.
On November 28, 1940, Romania formally joined the Axis Powers. The principal contribution was to supply Hitler with oil. The Ploesti oil fields were the main source of Germany's oil and gas until fields in the Ukraine and the Caucasus were captured in 1941-42, and those were not held long.
The Allies realized that a fatal blow could be dealt to the Germans if Ploesti could not produce. It was worth a dangerous mission by the United States Army Air Forces (USAAF) in August 1943. USAAF’s 8th and 9th Air Forces contributed B-24 groups that trained in the desert for operations against models of Ploesti. They were routed around Bucharest, which had heavy anti-aircraft defenses. Almost everything went wrong, with the lead plane and its precious navigator crashing before reaching Romania, to unexpectedly heavy fighters and flak over the target. Still, some damage was done to Ploesti, but continuous raids were needed. 54 planes were lost with 53 heavily damaged out of 177, making further follow-up raids impossible until April 1944. By the end of the war, shuttle raids had caused serious damage to Ploesti’s refineries and made oil production tenuous.
In February 1943, Red Army offensives north and south of Stalingrad cut through Romanian forces, causing many casualties. The cold and lack of transportation caused further fatalities. The German soldiers held the Romanians in contempt, not understanding that they were poorly armed, poorly fed, and led by incompetent, corrupt officers.
As the Red Army approached Bucharest, King Michael deposed Antonescu and joined the Allies. Russian troops entered Romania in August 1944. The Red Army immediately began recruiting men to fight against Germany.
Romania became a Soviet satellite after World War II, and was a signatory of the Warsaw Pact in 1955.