In 1140, Jamal ad-Din died, and Mu'in ad-Din continued to rule as regent for Jamal ad-Din's son
Mujir ad-Din Abaq. That year, Mu'in ad-Din besieged
Banias with help from King
Fulk of Jerusalem and Prince
Raymond of Antioch; Mu'in ad-Din offered 20,000 pieces of
gold per month to pay for their expenses. When it was captured, Mu'in ad-Din handed it over to Fulk and returned to Damascus. A more thorough alliance, to protect Damascus against Zengi, was negotiated during a visit by Mu'in ad-Din to
Jerusalem, accompanied by the future chronicler
Usamah ibn Munqidh. King Fulk died in 1143, and Zengi was assassinated in 1146. Zengi was succeeded by his sons
Saif ad-Din Ghazi I in
Mosul and
Nur ad-Din in Aleppo, and Mu'in ad-Din took the opportunity to besiege Baalbek; the governor,
Najm ad-Din Ayyub, father of
Saladin, quickly surrendered to him. Mu'in ad-Din also asserted control over
Homs and
Hama, and sent
Yarankash, the assassin of Zengi, to Nur ad-Din, after Yarankash sought refuge in Damascus. Mu'in ad-Din was always suspicious of Nur ad-Din's power, but it was his policy to remain on friendly terms with his neighbours wherever possible, whether they were Christian or Muslim. In 1147 Nur ad-Din and Mu'in ad-Din negotiated an alliance, in which Nur ad-Din married Mu'in ad-Din's daughter
Ismat ad-Din Khatun. Having established peace with Aleppo, Mu'in ad-Din set out to besiege
Salkhad and
Bosra, after their governor,
Altuntash, allied with Jerusalem against him. This broke Jerusalem's treaty with Damascus, forcing Mu'in ad-Din to turn to Nur ad-Din for assistance. Nur ad-Din arrived with the army of Aleppo, and the
crusaders were
forced to withdraw; both Bosra and
Sarkhad then surrendered to Mu'in ad-Din. In August 1147 Mu'in ad-Din was formally recognized as governor of Damascus by the
Caliph of
Baghdad Al-Muqtafi and the
Seljuk Sultan Mas'ud, and he was also recognized formally by the
Fatimid Caliph in
Egypt,
al-Hafiz. ==Second Crusade==