Why did the united states use chemical warfare during the vietnam war?

Guerilla Warfare tactics were used by Northern Vietnam forces during the Vietnam War. These tactics proved to be very effective against the alliance of Southern Vietnam forces with the US Army.

As a response to this, President John F. Kennedy authorized "Operation Ranch Hand", which consisted of deploying herbicides which caused forest defoliation. This action had the purpose of leaving the Vietnamese Guerillas without resources, as well as land that they could use in their advantage.

The aftermath of this operation was highly damaging to the environment, as well as causing health issues to millions of Vietnamese citizens.  

the answer is a) to be able to obey them.