In terms of game rules, I don’t have an answer, but in real-world terms military personnel aren’t the same thing as jeeps. Having Nation A use the troops of Nation B isn’t really Lend-Lease, it’s more along the lines of inter-allied cooperation, and that’s problematic for two reasons. Reason one (and WWII offers lots of examples in relation to the British and the Americans in 1944 and 1945) is that of senior military officers can be reluctant to serve under (or cooperate with) another nation’s senior military officers, Montgomery and Patton being a good case in point. Reason two is more subtle. Even with the best of intentions on everyone’s part, military forces from different nations generally can’t function as a single unit or even in close cooperation without a good deal of training for that specific purpose (as the short-lived ABDA found out in early 1942). Even when they speak the same language, different armies have different doctrines and practices, not to mention differences in nuts-and-bolts details like equipment and weapons and communication protocols. Even within the same nation, different services can find each other’s combat doctrine incomprehensible. The US Army and the United States Marine Corps in WWII sometimes ran into trouble because of this in the Pacific, one nasty example being the so-called “War of the Smiths” during the Marianas campaign in which a Marine General (Holland M. Smith) relieved an Army General (also named Smith) of command. During the Cold War, NATO devoted a lot of effort to the question of inter-operability (joint training exercises, standard small-arms ammunition calibers and so forth) for precisely these reasons.