Complex sentence: As of 2005, there were 1,100 M1 Abrams used by the United States Army in the course of the Iraq War, and they have proven to have an unexpectedly high level of vulnerability to roadside bombs.
Split into simpler sentences: As of 2005, there were 1,100 M1 Abrams used by the United States Army in the course of the Iraq War. They have proven to have an unexpectedly high vulnerability to roadside bombs.