Complex sentence: After the resignation of the king on 15 June 1917, Greece unified under a new king Alexander Constantine's son and the leadership of Eleftherios Venizelos, joined World War I on the side of the Allies.
Split into simpler sentences: After the resignation of the king on 15 June 1917, Greece unified under a new king, Alexander, Constantine I's son, and the leadership of Eleftherios Venizelos. It joined World War I on the side of the Allies.