Many blogs, discussion groups, and even Wikipedia assert that toward the end of Joseph Smith III's life he wavered in his belief that his father, Joseph Smith, Jr., did not teach or practice polygamy. According to a Wikipedia article on the origin of LDS polygamy, "In the end, Smith concluded that he was 'not positive nor sure that [his father] was innocent' and that if, indeed, the elder Smith had been involved, it was still a false practice." This position is used by those believing Joseph Smith, Jr. taught and practiced polygamy to diminish Joseph III's lifelong defence of his father's honor so their position can be made stronger. However, this position is just not true.
In 1913, during the last part of his 80th year and a little more than a year prior to his death, Joseph Smith III wrote the following in his memoirs:
It may be supposed at this writing (I am now nearing the close of my eightieth year), after the lapse of thirty-six years since I took the stand I did in Salt Lake City, among those people so diverse and hostile to my faith, that I should have some regrets that I had not adopted a different policy and followed, possibly, a more conciliatory course in order to make a more favorable impression. But I now record that such is not the case. Through all these years of reflection over the experiences which attended that first visit and several later ones, in which I preached at many places as opportunity offered--north, south, east, and west--and came in contact with many advocates of polygamy and plural marriage, I have entertained no wish that I had taken a less positive stand. I have never felt that any softer policy would have been wiser, for I am satisfied that I have found and occupied the only tenable ground open to me as the son of the prophet Joseph Smith, whom I believed to be innocent of responsibility for the evils they had embraced. As leader of the church reorganized, I felt sure that position was the one which could be maintained and successfully defended from the beginning of the conflict to the very end. (The Memoirs of President Joseph Smith III (1832-1914), p. 164, italics added)
Just a little over a year prior to his death, Joseph Smith III still believed his father "to be innocent of responsibility for the evils [polygamy] they [Utah LDS] had embraced." Thus, contrary to what many proclaim, he did not waiver in this position. He was firm in his belief to the end.
So, with all the information presented to him indicating his father's involvement in polygamy, how did he retain his firm belief of his father's innocence? Simply, his belief was founded in the truth because he could separate opinion from fact. As he said earlier in his memoirs:
I had made the law my study, and I had not regretted it, for I had already found that what knowledge along those lines I had acquired had been of much value to me in the work I had undertaken. I had gone into the conflict against error and false claims with a mind at least partially trained along legal lines, and I knew how to value evidence. I knew the difference between assumption and fact, and was prepared to examine whatever was presented as evidence in such a manner as to determine whether or not it was worth of the name of proof, especially in the controversy then existing between the various factions of the church. (ibid., p. 163)
Through all of his investigations, Joseph III never found a shred of credible evidence, even from the alleged plural wives, to prove his father guilty of polygamy. Thus, he remained convinced of his father's innocence to the end of his life. Indeed, Joseph Smith III was his father's greatest defender.