Complex sentence: In May 2010, President Barack Obama began a new chapter in the border mission calling once again on National Guard (army and air) troops, but this time only calling those troops in the border states for the mission.
Split into simpler sentences: In May 2010 President Barack Obama began a new chapter in the border mission calling once again on National guard (army and Air) troops. This time only calling those troops in the border states for the mission.