Question

president nixon sent us forces to cambodia in 1970 to
form a secret alliance with cambodian troops.
attack villages suspected of hiding viet cong fighters.
set up supply lines outside of vietnam.
bomb viet cong supply lines along the ho chi minh trail.

Explanation:

In 1970, Nixon sent US forces to Cambodia to bomb Viet Cong supply - lines along the Ho Chi Minh Trail.

Answer:

bomb Viet Cong supply lines along the Ho Chi Minh Trail.