Lyndon Johnson actually said...
I give you my pledge: We shall never let you down, nor your fighting comrades, nor the 15 million people of South Vietnam, nor the hundreds of millions of Asians who are counting on us to show here--here in Vietnam-that aggression doesn't pay, and that aggression can't succeed.
Context
Remarks to Members of the Armed Forces at Cam Ranh Bay, Vietnam.
10/25/1966