Complex sentence: ISIS's original aim was to establish a caliphate in the Sunni-majority regions of Iraq; following its involvement in the Syrian Civil War this expanded to include controlling Sunni-majority areas of Syria (see spillover from the Syrian War).
Split into simpler sentences: ISIS's original aim was to establish a caliphate in the Sunni-majority regions of Iraq. Following its involvement in the Syrian Civil War this expanded to include controlling Sunni-majority areas of Syria (see spillover from the Syrian War).