President Barack Obama declared a year ago that the use of chemical weapons by the Assad regime would cross a "red line" and could provoke U.S. military intervention. [Related: "Word in the News: Red Line."]

But what makes chemical weapons attacks more unacceptable than conventional military attacks, which have been raging in Syria since the start of the civil war there more than two years ago? And why is identifying the precise nature of last week's attack, including the chemical agents used, so difficult?

We spoke to chemical weapons expert Alexander Garza to get some answers.

A doctor and former Assistant Secretary for Health Affairs and Chief Medical Officer for the U.S. Department of Homeland Security, Garza is associate dean and