Complex sentence: The United States, alarmed at the incident, despatched Abbott Low Moffat on a special mission to Saigon and Hanoi to consider a negotiated referendum, however the realisation that the Viet-Minh would not accept compromise led to the US abandoning the idea.
Split into simpler sentences: The United States, alarmed at the incident, despatched Abbott Low Moffat on a special mission to Saigon and Hanoi to consider a negotiated referendum. However, the realisation that the Viet-Minh would not accept compromise led to the US abandoning the idea.