August 16. On this date in 1852, one day after a group of Bábís attempted to assassinate Naser al-Din Shah Qajar, Bahá'u'lláh was arrested and imprisoned. Years later, when he publicly revealed his mission, Bahá'u'lláh would claim that during this incarceration he was visited by the "Maid of Heaven" who told him his mission as the "Manifestation of God." Due to pressure from the Russian ambassador, Bahá'u'lláh was not executed but instead exiled to Baghdad along with the other Bábí leadership. Bahá'u'lláh traveled from Iran to Baghdad accompanied with a representative of the Russian legation and an entourage of Russian guards.

August 16. On this date in 1852, one day after a group of Bábís attempted to assassinate Naser al-Din Shah Qajar, Bahá'u'lláh was arrested and imprisoned. Years later, when he publicly revealed his mission, Bahá'u'lláh would claim that during this incarceration he was visited by the "Maid of Heaven" who told him his mission as the "Manifestation of God." Due to pressure from the Russian ambassador, Bahá'u'lláh was not executed but instead exiled to Baghdad along with the other Bábí leadership. Bahá'u'lláh traveled from Iran to Baghdad accompanied with a representative of the Russian legation and an entourage of Russian guards.

From Helen S. Goodall, a Disciple of `Abdu'l-Bahá, and Ella Goodall Cooper's "Daily Lessons Received at Akka: January 1908," "Exile of Bahá'u'lláh"...

When Bahá'u'lláh was imprisoned in Tihran, the Russian Ambassador went about twenty times to the Shah to plead in His behalf, saying, "This man has no fault. He is faultless. Why do you imprison Him?" The Shah answered that Bahá'u'lláh must be punished because He had ordered the attack on his life (see history). The Ambassador said, "That is not a reasonable supposition; for, if He had ordered such an attempt, He would have ordered a bullet put in the gun instead of merely powder and small shot." The Shah acknowledged this reasoning but was determined to hold Bahá'u'lláh responsible so as to have a pretext for keeping Him in prison.
Again and again the Russian Ambassador went to talk with the Shah about the matter, and at last the Shah confessed that he was afraid of the influence of Bahá'u'lláh, and that if he should set Him free, it would create a great tumult among the people.
The Ambassador answered, "If, then, you fear Him so much, why keep Him in Tihran? Would it not be better to exile Him to Baghdad?" This was accordingly done, and *an escort furnished of Cossack and Russian horsemen to protect Him from the Persian horsemen. *From Baghdad, Bahá'u'lláh was sent to Constantinople, then to Adrianople, then to Akka--by force He was sent to the place where He desired to be, thus fulfilling the prophecies of all the Holy Books. He came by His own Will. Had He simply appeared and declared Himself there, the opposers might have said, "Of course, He has read the prophecies and determined to appear in the Holy Land in order to mislead the people." But we see that He used the natural instruments who thought they were sending Him there by force. This is what we may call a real miracle.
After all, did the Shah accomplish his will, or did Bahá'u'lláh accomplish His Will?

From Nabil's The Dawn-breakers, "Chapter XXVI Attempt on the Shah's Life, and its Consequences"...

The Russian minister, as soon as he learned of the action which the government contemplated taking, volunteered to take Baha'u'llah under his protection, and invited Him to go to Russia. He refused the offer and chose instead to leave for Iraq. Nine months after His return from Karbila, on the first day of the month of Rabi'u'th-Thani, in the year 1269 A.H., Baha'u'llah, accompanied by the members of His family, among whom were the Most Great Branch and Aqay-i-Kalim, and escorted by a member of the imperial body-guard and an official representing the Russian legation, set out from Tihran on His journey to Baghdad.

In God Passes By, Shoghi Effendi alludes to the protection the Russian ambassador gave Bahá'u'lláh on different occasions, first after the attempted assassination of Naser al-Din Shah Qajar and again after the decision to exile Bahá’u’lláh from Iran, expressing his "desire to take Bahá’u’lláh under the protection of his government, and offered to extend every facility for His removal to Russia." In his Súriy-i-Haykal, Bahá’u’lláh included the Lawh-i-Malik-i-Rús, praising Czar Alexander II of Russia in these terms:

when this Wronged One was sore-afflicted in prison, the minister of the highly esteemed government (of Russia)—may God, glorified and exalted be He, assist him!—exerted his utmost endeavor to compass My deliverance. Several times permission for My release was granted. Some of the ‘ulamás of the city, however, would prevent it. Finally, My freedom was gained through the solicitude and the endeavor of His Excellency the Minister. …His Imperial Majesty, the Most Great Emperor—may God, exalted and glorified be He, assist him!—extended to Me for the sake of God his protection—a protection which has excited the envy and enmity of the foolish ones of the earth.