How did the beautiful, peaceful mountain meadows of the Utah Territory become the killing fields of The First 9/11 in America?

The leaders of the Mormon church knew and so did these men: John D. Lee, George W. Adair, Jr., William C. Stewart, Nephi Johnson, Ira Hatch, Samuel Jukes, Jacob Hamblin, William H. Dame, Elliott Wilden, David W. Tullis, Daniel Hammer Wells, and dozens more. All of these men knew But the Arkansas and Missouri members of the Baker-Fancher wagon train did not know until it was too late