After the Kanesville camp was made, Mormon leaders were immediately concerned over two major problems: sending an advance company to the Rocky Mountains, and locating a place for the main portion of the camp to build winter quarters until they, too, could go west in the spring.
On July 1st, the first problem was solved by the process of elimination. On that day, Captain James Allen of the U.S. Army's First Regiment of Dragoons of Fort Leavenworth rode into the Mosquito Creek camp with a request from President James K. Polk for a battalion of 500 Mormon men to fight in the Mexican War. Part of the agreement was that the Mormons would be permitted officially to camp on Potawatomi lands and, unofficially, allowed to move across the Missouri River and settle temporarily on Omaha lands, which had previously been closed to whites. (It was important to the Mormons to put one more river between them and the "anti-Mormons.")
Mormons were very apprehensive about Captain Allen's proposal. Abraham Day was with his family at Council Bluffs when he heard the news. He wrote: "Here is one man who will not go, dam'um." However, after Brigham Young's talk to the brethren the following day, Abraham Day volunteered.
According to John Taylor, Brigham Young said he had been "...trying to effect [federal aid] for several years, and this move had been made a little too quick for us ...Supposing we were to refuse this offer; we would have to go to California and have to depend upon our own resources to fight, when if we embrace this offer we will have the US to back us and have an opportunity of showing out loyalty and fight for the country that we expect to have our homes. If we did not go and help take it, what would be said when we got there and settled down. It would be as it always had been, get out of the way Mormons, get out of the way. Our fathers and us fought for the liberties of this country and we are the only citizens. Whereas if we go and help take the country we will at least have the right, and I do not want any body to be in these wildernesses and undiscovered before we are. I think the President had done us a great favor by calling upon us. It is the first call that has been made upon us that ever seemed likely to benefit us. Now I want you young men to go and all that can go, young or married, I will see that their families are taken care of; they shall go on as far as mine, and fare the same..."
The battalion of about 489 men, some 20 wives who went as laundresses, four to a company, and perhaps a dozen boys as officers' aids, (because of imperfect records the exact number of men, women, and children in the battalion is not known and estimates vary) was eventually raised. They mustered in a little south of what became Council Bluffs, near what is now the Iowa School for the Deaf and on July 20th, the new recruits started off for Fort Leavenworth, 150 miles down the Missouri.
There is still, as there was then, a widespread belief among Mormons that raising the Mormon Battalion was a great sacrifice on the part of the church to an undeserving government. Actually, the government was responding to the requests of Mormon leaders for any kind of help in their move to the west. The Mormon leaders provided the men because it would help demonstrate their loyalty to their country (during wartime, Americans, including Mormons, generally work together), and because the church would benefit materially from the military pay, from the arms that the men could keep, from the uniform money allotments (since Mormons were allowed to wear their own clothes), and from the fact that many men would be transported west at government expense. (This last point was only partly realized, because, after being discharged, members of the battalion had to transport themselves back from California to either Salt Lake City or Winter Quarters to pick up or meet with their families.)Source: Historic Resource Study - Mormon Pioneer National By Stanley B. Kimball, Ph.D., May 1991. (The study focuses on the history of the trail from its official beginning in Nauvoo, Illinois, to its terminus in Salt Lake City, Utah, during the period 1846-1869. During that time, thousands of Mormon emigrants used many trails and trail variants to reach Utah. This study emphasizes the 'Pioneer Route' or 'Brigham Young Route' of 1846-1847. The sections on Mormon beliefs and motivations for going west have been omitted. Interested persons can find ample sources for that information. The footnotes, bibliography, maps, pictures, pioneer companies by name and dates for the 22-year period, and historic sites - about 2/3 of the book - have also been left out for space considerations. Thanks to Dr. Kimball and the National Park Service for the availability of this information.)