Complex sentence: The British, who were earlier reluctant to colonise Assam, came into direct contact with a belligerent Burmese occupying force; and following the First Anglo-Burmese War annexed not just the Ahom kingdom but also Burma.
Split into simpler sentences: The British, who were earlier reluctant to colonise Assam, came into direct contact with a belligerent Burmese occupying force. Following the First Anglo-Burmese War they annexed not just the Assam but also Burma.