Firstly, I would like to thank "Tac", he pointed out that the Red Army invaded Manchuria to fight the Japanese at the end of WW2. Seriously, I'm a history major. How the hell did I overlook that!?
First let's start with the most obvious
Tank Dempsey: Was dispatched to Wittenau Sanitorium(Verruckt) in order to extract Dr. Peter M. but was captured. Dr. Richtofen was suspicious of Dr. Peter so had him watched closely and the soldiers at Wittenau became aware that a highly trained soldier(Marine) was headed there. Trap sprung and Tank captured for questioning-but Richtofen used him instead.
Nikolai Belinski:Feared soldier of the Red Army, he was always forced to fight on the front lines. When the USSR invaded Japanese held land in 1945 intelligence(German/935 or Japanese) realized that this highly valuable killing machine would be in Manchuria and so elite forces were sent to intercept him. Takeo Masaki- having earned his name and rank as Japan's own fearless god of warfare was one of those sent to capture him.
Takeo Masaki: Having successfully captured the feared Nikolai, Takeo was one of the few(if not only) Japanese soldiers left, the others being elite German soldiers(likely SS) working under Group 935. Since he was, he was also seen as an asset to Richtofen as he was a killing machine like Nikolai and if it were not for him, Nikolai wouldn't have been captured. So Takeo was also taken into custody as the newest test subject, his prison cage was near that of the first known subject for Maxis's "barrier" experiments. Mex.
"Mex""The Mexican": "He was, EL BURRO!!!" the song playing from Mule Kick. During the "Christo War" that was the last Mexican conflict before WW2 it is likely that Mex was a Vaquero that fought on the side of justice(at least what he thought) and was so excellent at combat that his exploits were almost never exaggerations(imagine Clint Eastwood, Mexican version). When the Mexicans declared war on the Axis powers in 1942 it so happens that Mex signed up for the fight(at this point being a bit too old but it was really for PR) but was then captured by elite German forces when 935(Maxis specifically) told the German high-command they wanted him to experiment on in order to break through the "barrier" and control their experiments.
The experiments on Mex lead to his horrible death(involving evisceration). Takeo subverted to the way he was raised-quiet, only concerned with his honor and ancient proverbs to live by. Nikolais despair over his tremendous physical torture lead to his depression which he could not recover from without the induction of something from his past-alcohol. And Tank had to be experimented on(without torture whatsoever) because torture only hardened him. When the experiments eventually wiped his memory he still knew one thing- hatred. As such Dr. Richtofen re-wiped the memory of Tank and wiped the memory of nikolai and Takeo when his plans fell apart.
This, is how the 4 soldiers from around the world became the 3 "super soldiers", fighting with the one man who had a plan. Did he plan on destroying the zombies and leading them to victory for their countries? Hopefully. They only knew that the Dr. was familiar, that he did not seem to want to hurt them, and that in order to survive they had to trust him.
Sorry for the length. It's better to go into unnecessary detail than have many questions.