What did he say the war was trying to stop ?

The war was trying to stop Communism.

They didn't know the first thing about Diem's tyranny, or the nature of Vietnamese nationalism, or the long colonialism of the French—this was all too damned complicated, it required some reading—but no matter, it was a war to stop the Communists, plain and simple, which was how they liked things, and you were a treasonous pussy if you had second thoughts about killing or dying for plain and simple reasons.