Ok, I may be just a tad late in posting lol. And I don't think I can type you up a sample essay but instead a plan?
Basically the question is asking you why the americans could not separate Nationalism from communism.
Reasons:
Pride/prestige- Pentagon Papers released in 67 (i think) revealed they DID NOT want to lose another war to some stupid asian country like they did to N. Korea
Communism- They say that they are preventing the spread of communism (containment, truman doctrine, etc, which they are) however they also wanted to preserve American interests. For example if S. Vietnam were to become Communist then no more trading would happen between Vietnam and America. And the US love their rice, rubber, etc, as it guarantees them a more prosperous economy, (the greedy bastards that they are)
The two reasons are so intertwined together that they can not be separated in this way.
I can clearly see that this would be a 15-25 mark response as it uses one of those hard words at the start of the sentence- not 'explain', 'discuss', 'analyse', etc, but a 'to what extent' and in this case, 'evaluate' question. I personally love 'evaluate' questions because it is asking you specifically to make a judgement, on whether the statement is correct or not. You would also need supporting sources and/or quotes to support this argument.
Hope my advice helps! (and it's not too late)
GL!