Complex sentence: When Baibars mounted a counter-offensive from Egypt on November 12th, the Mongols had already retreated beyond the Euphrates, but these unsettling events allowed Edward to negotiate a ten year peace treaty with the Mamluks.
Split into simpler sentences: When Baibars mounted a counter-offensive from Egypt on November 12th, the Mongols had already retreated beyond the Euphrates. However, the events did allow Edward to negotiate a ten year peace treaty with the Mamluks.