I just heard of this a few days ago. Its sad that a lot of crap has happened in US history that is not taught in US History classes. This attrocity is not without precedent. In the very same county 25 years prior a similar atrocity was committed by Governor Lilburn W. Boggs and much of the population of Jackson County Missouri.

GOVERNOR BOGGS' ORDER OF EXTERMINATION

"Headquarters of the Militia,"
City of Jefferson, Oct. 27, 1838.

"General John B. Clark:

"Sir Since the order of this morning to you, directing you to cause four hundred mounted men to be raised within your division, I have received by Amos Reese, Esq., of Ray county, and Wiley C. Williams, Esq., one of my aids, information of the most appalling character, which entirely changes the face of things, and places the Mormons in the attitude of an open and avowed defiance of the laws, and of having made war upon the people of this state. Your orders are, therefore, to hasten your operation with all possible speed. The Mormons must be treated as enemies, and must be exterminated or driven from the state if necessary for the public peace--their outrages are beyond all description. If you can increase your force, you are authorized to do so to any extent you may consider necessary. I have just issued orders to Maj. Gen. Willock, of Marion county, to raise five hundred men, and to march them to the northern part of Daviess, and there unite with Gen. Doniphan, of Clay, who has been ordered with five hundred men to proceed to the same point for the purpose of intercepting the retreat of the Mormons to the north. They have been directed to communicate with you by express, you can also communicate with them if you find it necessary. Instead therefore of proceeding as at first directed to reinstate the citizens of Daviess in their homes, you will proceed immediately to Richmond and then operate against the Mormons. Brig. Gen. Parks of Ray, has been ordered to have four hundred of his brigade in readiness to join you at Richmond. The whole force will be placed under your command.

I am very respectfully,
your ob't serv't,
"L. W. Boggs,
Commander-in-Chief."

source

This order was not lifted from the books until 1976 when Governor Christoher S. Bond rescinded the order and formally apologised to the mormons in behalf of the state of Missouri.

One of the worst events connected to the extermination order:

On October 30, 1838, segments of the Missouri militia attacked a settlement of Latter-day Saints at Jacob Haun's mill in Caldwell County, Missouri. Because the attack was unprovoked in a time of truce, had no specific authorization, and was made by a vastly superior force with unusual brutality, it has come to be known as "The Haun's Mill Massacre." Tensions in Missouri between Latter-day Saints and Missourians led to Governor Lilburn W. Boggs's October 27 Extermination Order demanding that Latter-day Saints leave the state or be exterminated. Thirty to forty LDS families were at Haun's Mill when some 200 to 250 militia marched against the village. The residents assumed an earlier truce still held and were surprised by the attack. Women and children fled across a stream into the woods, while the men gathered in the blacksmith shop, but found it a poor place for defense because the Missourians were able to fire through the widely spaced logs directly into the group inside. Seventeen Latter-day Saints and one friendly non-Mormon were killed. Another thirteen were wounded, including one woman and a seven-year-old boy. Some deaths were particularly offensive: Seventy-eight-year-old Thomas McBride surrendered his musket to militiaman Jacob Rogers, who shot him and then hacked his body with a corn knife; William Reynolds discovered ten-year-old Sardius Smith hiding under the bellows and blew the top of the child's head off. The dead were thrown into an unfinished well and lightly covered with dirt and straw. No Missouri militiamen were killed, though three were wounded. A few Missourians returned the next day and took plunder. The Haun's Mill Massacre was largely ignored in judicial proceedings investigating the Missouri conflict with the Latter-day Saints, but it became embedded in the LDS psyche as an epitome of the cruel persecutions that they had endured.

source

In 1839 when mormon founder Joseph smith pled his case to President Martin Van Buren this was the response "your cause is just, but I can do nothing for you. If I take up for you I shall lose the whole state of Missouri". So rather than risk losing reelection by angering the citizens of Missouri he simply ignored the atrocity.

-Steve G.