Complex sentence: In 720 BC he sent an army in support a rebellion against Assyria in Philistia and Gaza, however Piye was defeated by Sargon II, and the rebellion failed.
Split into simpler sentences: In 720 BC, he sent an army in support a rebellion against Assyria, which was taking place in Philistia and Gaza. However, Piye was defeated by Sargon II and the rebellion failed.