Complex sentence: Following the U.S.-launched 2003 invasion of Iraq, the situation deteriorated, and by 2007, the intercommunal violence between Iraqi Sunni and Shi'a factions was described by the National Intelligence Estimate as having elements of a civil war.
Split into simpler sentences: Following the U.S.-launched 2003 invasion of Iraq, intercommunal violence between Iraqi Sunni and Shi'a factions became prevalent. The conflict escalated over the next several months until by 2007, the National Intelligence Estimate described the situation as having elements of a civil war.