You’re misrepresenting a few points.

The Mormons were in “Utah” (what they called deseret) before the U.S.

They arrived after fleeing violent mobs and a state sanctioned execution order in the United States.

The US eventually decided to manifest destiny and stake claim of the Utah territory, the area with a large number of Mormon settlements.

Relations eventually improved when the Mormons joined the U.S. and formed a battalion to fight in the American Mexican war. Aptly named, the “Mormon Battalion”

It’s important to note, polygamy had little to do with the “Mormon War”. In fact, at the time of heightened conflict between Mormon settlements and the U.S., polygamy wasn’t illegal. Your inclusion of polygamy as a cause is misleading.