The two main reasons why Douglass opposed John Brown in his plan to raid Harpers ferry is because first he was sure that the plan would fail and thus it would lead to many black deaths. The second reason is that Douglas thought that this plan would hurt the abolition movement due to a backlash.
As noted by Douglass in his autobiography written later in life:
1. Douglass feared the raid was a suicide mission, doomed to fail. He said Brown's plan was "was going into a perfect steel-trap, and that once in he would never get out alive; that he would be surrounded at once and escape would be impossible."
2. Douglass said he preferred a less explosive approach, going with a previous plan they had devised to set up a network of encampments in the Allegheny Mountains and through such camps help slaves from the surrounding areas to flee. Douglass said that Brown was "f<span>or Harper’s Ferry, and I against it; he for striking a blow which should instantly rouse the country, and I for the policy of gradually and unaccountably drawing off the slaves to the mountains, as at first suggested and proposed by him."
Incidentally, some scholars have questioned whether Douglass's account in his 1882 autobiography tells the full story as to what happened in the lead-up to Harpers Ferry in 1859. Theodore Hamm, writing about "When Frederick Douglass Met John Brown" (Jacobin magazine, issue 29, 1968), presented evidence that Frederick Douglass ultimately had helped to rally support for the raid, but that friends in the media helped keep Douglass's name out of any stories about the raid. The same media reported that at his trial, John Brown showed "</span><span>determination to implicate no others even to save himself" (as observed by the Weekly Anglo-African newspaper in New York).</span>
