My friends an I were playing a&a 1941 and soviet union wanted to mobilize units in India. Is that ever allowed? If yes, when? I read through the whole book and I did not find anything about when you can do that.
You may only mobilize units in a territory you (not your ally) have owned since the start of the turn. Additionally, you liberate allied territories when you capture them, immediately returning ownership to the original owner*. Thus the USSR cannot mobilize in India, since it is owned by the UK, not the USSR.
* - Details, it is technically possible to create a situation where the USSR could mobilize in India. It would require that the Axis captures both the UK capital, and India, then the USSR recapture India (a territory is not liberated if the owner's capital is in enemy hands), then on the following turn (after a full turn of ownership) the USSR may mobilize in India.
You can only mobilize in an area which 1) you control and 2) on which there is an industrial complex.
In the beginning of the game, India does not have an industrial complex. Nor does Russia control it. The "best" Russia can do is to advise her UK ally to 1) build an industrial complex in India, then 2) build three pieces and 3) move them into Novosibirsk, or some other place of Russia's choice. Each of those activities takes up a whole turn.
As long as the UK (an ally) holds India, Russia cannot capture or control it. But suppose Japan 1) captures it and 2) builds an industrial complex and 3) allows Russia to "recapture" it. Then what happens?
Normally, Russia will have to return India to the UK, an ally. The exception to the rule is if the UK's capital is under occupation, say by Germany. Then since it is basically out of the game, Russia can keep India, build an industrial complex there, and mobilize the following turn.