Fulk was then faced with a new and more dangerous enemy: the atabeg Zengi of Mosul, who had taken control of Aleppo and had set his sights on Damascus as well; the union of these three states would have been a serious blow to the growing power of Jerusalem. A brief intervention in 1137–1138 by the Byzantine emperor John II Comnenus, who wished to assert imperial suzerainty over all the crusader states, did nothing to stop the threat of Zengi; in 1139 Damascus and Jerusalem recognized the severity of the threat to both states, and an alliance was concluded which halted Zengi's advance. Fulk used this time to construct numerous castles, including Ibelin and Kerak. After the death of both Fulk and Emperor John in separate hunting accidents in 1143, Zengi invaded and conquered Edessa in 1144. Queen Melisende, now regent for her elder son Baldwin III, appointed a new constable, Manasses of Hierges, to head the army after Fulk's death, but Edessa could not be recaptured, despite Zengi's own assassination in 1146. The fall of Edessa shocked Europe, and a Second Crusade arrived in 1148.