If the country you live in declares war on your home country, who would you support?
Who ‘would’ you support is not the same as who ‘should’ you support.
For the very specific situation you described, who should you support is the US.
You are a citizen and citizenship are philosophical and political processes. So it begs the question of why did you chose to become a US citizen to start the internal debate. Your arbitrarily granted US citizenship demands that you sever all ties to your past philosophical and political allegiances. It does not matter how many yrs you lived in your previous country. The oath of citizenship demands and expects immediate severance.
Of course, there is no way for anyone to verify that, so all we have are a public oath and your own conscience holding you to that oath.
If you chose to support your previous country, under your US citizenship, that would make you a traitor in every perspective of the charge. So for the very specific situation you described, you would have to ask yourself if being called a traitor by other US citizens is worth being called a patriot by citizens from your country of origin.