"Hi China, so umm, we are going to invade Russia from Mongolia. As anyone in the right state of mind would know it's gonna be a logistical nightmare. So we suppose we have to ask: can we borrow your Tianjin port and your railroad system, so we can transport stuff into Mongolia? Also don't shoot when our convoys fly over your sovereign territory. xoxo"

(Re: Tianjin port, I think Mongolia exports most of its not-China-destined coals and resources through that, which is why as much as they pretend to look for a "third neighboring nation" they can't really afford to irk China too much)