Johnson committed troops to Vietnam on the basis that the South Vietnamese government could not handle the Viet Cong. Further, he wanted to prevent a communist takeover.

Bombing:

Bombed Viet Cong strongholds and supply routes (Ho Chi Minh Trail)
Bombing killed thousands of people and turned Vietnamese people against the US

Search and destroy:

US soldiers would surround villages and seek out Viet Cong forces
Raids were based on poor information and often led to the killing of Vietnamese civilians

Chemical weapons:

There were two main types: agent orange (poisonous weed killer to defoliate the jungle) and Napalm (would burn the jungle where the Viet Cong were located)