The Mormons knew for at least a week, maybe more, that the Fancher Party was not the the U.S. army trying to invade. After the Fancher party held off the Mormons for four days, the Mormons deceived them under a flag of truce promising safe passage, split up the survivors, then murdered most of them.
OK, the Fancher party did not have the pass that Brigham Young decreed to travel through Utah. Except that decree was not made public until September 15, when the Massacre (under the flag of truce and promise of safe passage) was over on Sept. 11, four days after they were dead.
How can you cloak yourself in religious piety, accuse others of religious bigotry, while justifying a massacre of innocent people by ignoring the facts?
Mountain Meadows was wrong.
That said, how much do you know about why the Mormons were in Utah to begin with?
Why did the people in Mountain Meadows see the wagon train something to fear?
That could be asked of many posts addressed to him that never get a response.
It’s a Millet to Missionary teaching...