Complex sentence: On September 16, Iraqi forces began the offensive to dislodge ISIL from the border with Syria to the south of the Euphrates, also aimed at tightening Iraq's hold on its border with Syria.
Split into simpler sentences: On September 16, Iraqi forces began the offensive to dislodge ISIL from the border with Syria to the south of the Euphrates. The offensive was also aimed at tightening Iraq's hold on its border with Syria.