Complex sentence: The Irish War of Independence was a guerrilla campaign by the IRA in support of the Irish Republic declared in 1919 and entered its bloodiest phase in the first six months of 1921.
Split into simpler sentences: The Irish War of Independence was a guerrilla campaign by the IRA in support of the Irish Republic, which was declared in 1919. The war and entered its bloodiest phase in the first six months of 1921.