Sojourn in Baghdád

A twenty-year-old Faith had just begun to recover from a series of successive blows when a crisis of the first magnitude overtook it and shook it to its roots. Neither the tragic martyrdom of the Báb nor the ignominious attempt on the life of the sovereign, nor its bloody aftermath, nor Bahá'u'lláh's humiliating banishment from His native land, nor even His two-year withdrawal to Kurdistán, devastating though they were in their consequences, could compare in gravity with this first major internal convulsion which seized a newly rearisen community, and which threatened to cause an irreparable breach in the ranks of its members. 
More odious than the unrelenting hostility which Abú-Jahl, the uncle of Muhammad, had exhibited, more shameful than the betrayal of Jesus Christ by His disciple, Judas Iscariot, more perfidious than the conduct of the sons of Jacob towards Joseph their brother, more abhorrent than the deed committed by one of the sons of Noah, more infamous than even the criminal act perpetrated by Cain against Abel, the monstrous behavior of Mírzá Yahyá, one of the half-brothers of Bahá'u'lláh, the nominee of the Báb, and recognized chief of the Bábí community, brought in its wake a period of travail which left its mark on the fortunes of the Faith for no less than half a century. 
This supreme crisis Bahá'u'lláh Himself designated as the AyyÁM-i-Shidád (Days of Stress), during which "the most grievous veil" was torn asunder, and the "most great separation" was irrevocably effected. It immensely gratified and emboldened its external enemies, both civil and ecclesiastical, played into their hands, and evoked their unconcealed derision. It perplexed and confused the friends and supporters of Bahá'u'lláh, and seriously damaged the prestige of the Faith in the eyes of its western admirers. 
It had been brewing ever since the early days of Bahá'u'lláh's sojourn in Baghdád, was temporarily suppressed by the creative forces which, under His as yet unproclaimed leadership, reanimated a disintegrating community, and finally broke out, in all its violence, in the years immediately preceding the proclamation of His Message. It brought incalculable sorrow to Bahá'u'lláh, (Source)