Romania had its first involvement in the war in providing transit rights for members of the Polish government, its treasury, and many Polish troops in 1939. During 1940, threatened with Soviet invasion, Romania ceded territory to the Soviet Union, Hungary, and Bulgaria, and following an internal political upheaval, Romania joined the Axis. Subsequently, the Romanian army participated with over 600,000 men in the German-led invasion of the Soviet Union, with its forces taking part in the capture of Odessa, Sevastopol and ultimately suffering irrecoverable losses at Stalingrad. Romania was also a major source of oil for Nazi Germany via the Ploiești oil fields.