I don't know where to begin, really, so I guess I'll start with the questions I posited in the title to this and this one: how was Romania able to mobilize so many men during WW2 in support of Germany and why/how did they do so?
It's hard to find much about Romania (in English) during WW2 beyond the basic factual statements that Romania contributed almost a million men to the German war effort and some basic calculations regarding their impetus to do so.
So, I'm curious how this all played out: why Romania in such a prominent role (i.e.: why not Bulgaria? Or Serbia? Or Albania? What made Romania special, if anything?); Did they use their own materiel or were they equipped by Germany?; Did they fight well compared to the other Axis allies of Germany?
Why rely on the Romanians to guard the flanks of Stalingrad? How could Italian contributions be so limited in terms of fighting ability and mobilization, yet Romanian contributions apparently so helpful and (apparently) eager? There simply isn't much on this topic that I can find to satisfy my curiosity.
There were several factors why Romania was such a fervent Axis member. The following quote from the US governments country study from 1990 says it quite well. "morbid competition with Hungary to curry Hitler's favor... [in hope of]... regaining northern Transylvania." U.S. government Country study: Romania, c. 1990.
Romania lost in 1940 most of the territorial gains from the first world war were lost due to a Soviet Ultimatum for Bessarabia and the second Vienna award in which Northern Transylvania was lost. under the Treaty of Craiova, Southern Dobruja was also lost to Bulgaria.
Hereafter the Iron Guard came to power in Romania which was a strong Axis leaning power. Since Romania got a chance to gain their lost regions back from the Soviet Union and promises of more made their military involvement so high. Also because Romania and Hungary were rivals vying for Hitlers favour, they tried to outdo eachother. It is described nicely in Cristian Craciunoiu; Mark W. A. Axworthy; Cornel Scafes (1995). Third Axis Fourth Ally: Romanian Armed Forces in the European War, 1941-1945. London: Arms & Armour.
The same book also describes the composition of Romanian arms. Most of it was bought from Germany. Romania had a great position to bargain with the Axis powers due to their oil.
I do not have a reference for it but I remember reading that Hitler and his generals did not expect a counterattack from the Soviets, which is why the put their allies on the flanks at Stalingrad so the Germans could use their superior troops for the push forward.