I don't think Myanmar will let go those oil reserves. No one can impose anything on Myanmar, just see the name China. That's the extend this discussion by US can go. Might as well as add Russia to it.
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Russia does not have the same interest in Myanmar as China does. Anyway it will look like a hypocrite if it goes on about "internal affairs" after it annexed Crimea from Ukraine lol
China will not fight for Myanmar. If BD was to invade and annex the whole of Arakan, I doubt that it will even downgrade relations with BD to be honest. If China is ever to become a developed country, it needs to bring a large chunk of humanity on it's side and putting it's neck on the line for Myanmar won't help in this endeavour.
Think about the countries in Asia that are already enemies of China - India, S Korea, Japan, Vietnam. Does China keep wanting to add more just to please Myanmar? By intervening to save Mynamar it won't be just BD that it will piss off but many more as well. At the end of the day, what real choice does Myanmar have but to go with China anyways? These oil and gas pipelines lead into China. Without the Chinese market, they may as well not exist. Myanmar needs China just like China needs Myanmar.
The real power in the equation is BD. If BD had a government that had the initiative to deal with Myanmar, then this situation would not have arisen in the first place. BD weakness has emboldened Mynamar.