If war were to break out between Russia and USA, would Russian Mormons fight against the USA? My understanding about Mormon beliefs is that they do serve in armed forces in the country where they live.

Also I may be incorrect but, looking at this question, it seems that Mormons hold the United States in an especially high regard among nations. This can also be seen from the talk "This Nation Shall Endure" (by Ezra Taft Benson, President of the Quorum of the Twelve Apostles, in 1973) linked to the accepted answer to the question.

So the question I wanted to ask is about Mormons living in Russia (where, according to this page, the church has 23,180 adherents).

Edited to a single question:

If a war broke out between Russia and the USA, would Mormons in Russia be instructed to not to fight, considering perhaps reasons such as that it could destroy the United States constitution?
Latter-day Saints abide the law of the land in which they live; honorably answering a call when conscripted for military service is part of this belief. Volunteer military service is also respected as a way of serving one's country. Latter-day Saints should live in accordance with the revealed will of the Lord and according to their own conscience.


  Latter-day Saints in the military do not need to feel torn between their country and their God. In the Church, “we believe in being subject to kings, presidents, rulers, and magistrates, in obeying, honoring, and sustaining the law” (Articles of Faith 1:12). Military service shows dedication to this principle.


(From https://www.lds.org/topics/war)

This happened in WW2. There's even a movie based on a true account about this ("Saints and Soldiers"). War is awful, but Mormons accept this reality where they do not have the power to avoid it.

Further reading: http://www.mormonnewsroom.org/article/military-service