Complex sentence: Meanwhile, the Seljuq Zengi, Atabeg of Mosul, had added to his rule in 1128 Aleppo, the key to power in Syria, contested between the rulers of Mosul and Damascus.
Split into simpler sentences: Meanwhile, the Seljuq Zengi, Atabeg of Mosul, had added Aleppo to his rule in 1128. Aleppo was the key to power in Syria, contested between the rulers of Mosul and Damascus.