How would Joseph Smith know about the destruction in Jackson county?

B. H. Roberts reports another prophecy of Joseph Smith:

The following prophetic incident is given upon the authority of Mr. Leonidas M. Lawson, now of New York City, formerly a resident of Clay county, Missouri, and a brother-in-law of General Doniphan. “In the year 1863,” says Mr. Lawson, “I visited General A. W. Doniphan at his home in Liberty, Clay county, Missouri. This was soon after the [Civil War] devastation of Jackson county, Missouri under what is known as ‘Order No. 11.’ This devastation was complete. Farms were everywhere destroyed, and the farm houses were burned. During this visit General Doniphan related the following historical facts and personal incidents.” Then follows in Mr. Lawson’s account a recital of the treatment meted out to the Saints in Missouri from the time of their first arrival in 1831, to their expulsion, including recitals of the personal relations of General Doniphan and Joseph Smith, including the following incident which occurred during the Prophet’s imprisonment in Liberty jail:
“On one occasion General Doniphan caused the sheriff of the county to bring Joseph Smith from the prison to his law office, for the purpose of consultation about his defense. During Smith’s presence in the office, a resident of Jackson county, Missouri, came in for the purpose of paying a fee which was due by him to the firm of Doniphan & Baldwin, and offered in payment a tract of land in Jackson county.
“Doniphan told him that his partner, Mr. Baldwin, was absent at the moment, but as soon as he had an opportunity he would consult him and decide about the matter. When the Jackson county man retired, Joseph Smith, who had overheard the conversation, addressed General Doniphan about as follows:
“‘Doniphan, I advise you not to take that Jackson county land in payment of the debt. God’s wrath hangs over Jackson county. God’s people have been ruthlessly driven from it, and you will live to see the day when it will be visited by fire and sword. The Lord of Hosts will sweep it with the besom of destruction. The fields and farms and houses will be destroyed, and only the chimneys will be left to mark the desolation.’
“General Doniphan said to me that the devastation of Jackson county forcibly reminded him of this remarkable prediction of the Mormon Prophet. . . .
In a letter from Mr. A. Saxey of Spanish Fork, Utah to Mr. Junius Wells treating further of the fulfillment of this prophecy, so well attested, Mr. Saxey under date of August 25, 1902 says:
“In the spring of 1862 my regiment went south, and it was during that time that “Order No. 11″ was issued, but I was back there again in 1864, during the Price raid, and saw the condition of the country. The duty of executing the order was committed to Colossians W. R. Penick’s regiment, and there is no doubt but that he carried it into effect, from the howl the copperhead papers made at the time. I went down the Blue river, we found houses, barns, outbuildings, nearly all burned down, and nothing left standing but the chimneys which had, according to the fashion of the time, been built on the outside of the buildings. I remember very well that the country looked a veritable desolation.”
– B. H. Roberts, New Witnesses for God , Vol. 1, p. 298

How did Joseph Smith manage to prophecy so specifically?

See:

Jeff Lindsay – Mormon Answers: Fulfilled Prophecies of Joseph Smith

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