Well, to be fair to Romania, it was not Hitler's ally by its own volition.
How's that?
In 1940, Romania's territorial gains made following World War I were largely undone. In July, after a Soviet ultimatum, Romania agreed to give up Bessarabia and Northern Bukovina. Two thirds of Bessarabia were combined with a small part of the Soviet Union to form the Moldavian Soviet Socialist Republic. The rest (Northern Bukovina, northern half of the Hotin county and Budjak) was apportioned to the Ukrainian Soviet Socialist Republic. Shortly thereafter, on 30 August, under the Second Vienna Award, Germany and Italy mediated a compromise between Romania and the Kingdom of Hungary: Hungary received a region referred to as "Northern Transylvania", while "Southern Transylvania" remained part of Romania. Hungary had returned all of Transylvania after World War I in the Treaty of Trianon. On 7 September, under the Treaty of Craiova, Southern Dobruja (which Bulgaria had lost after the Romanian invasion during the Second Balkan War in 1913), was ceded to Bulgaria under pressure from Germany.
I think this clears any doubt about "own volition".
Also I was replying to Bryant comment and his line:
"
As long as the Eastern Europeans are OK with the American military presence on their soil, I am not complaining. "
edit:
added this:
On 8 October German troops began crossing into Romania. They soon numbered over 500,000.
On 23 November Romania joined the Axis powers.
Although Wikipedia is wrong about this , the troops started crossing into the new Romanian borders only at the begging of November , but still before romania actually joined the axis.