Complex sentence: Late 2011 marked growing influence of the Islamist group al-Nusra Front within the opposition forces, and in 2013 Hezbollah entered the war in support of the Syrian army.
Split into simpler sentences: Late 2011 marked growing influence of the Islamist group al-Nusra Front within the opposition forces. In 2013 Hezbollah entered the war in support of the Syrian army.