Complex sentence: He launched successful military campaigns against the nearby Moslem rulers, but his conflict with Joscelin I of Courtenay enabled Imad ad-Din Zengi to secure Mosul and Aleppo.
Split into simpler sentences: He launched successful military campaigns against the nearby Moslem rulers. However, his conflict with Joscelin I of Courtenay enabled Imad ad-Din Zengi to secure his position in both Mosul and Aleppo.