Why Are Chemical Weapons a 'Red Line' for US Intervention?

The U.S. military carried out air strikes against the forces of Syrian President Bashar al-Assad on Thursday. President Donald Trump has made it clear the strikes were a retaliation for Assad's suspected use of chemical weapons April 4, which left about 100 dead, including women and children. But why were the use of chemical weapons a so-called "red line" for Trump? VOA's Bill Gallo examines.