I think it's pretty hard to maintain the view that WW1 and previous wars were any more 'honourable' than later ones based on one or two moments of truce occuring. In modern wars, military concerns are often overridden in order to protect civilians - whereas civilians were often deliberately targetted in earlier ones.
Vietnam was perhaps a particularly brutal war, but I see it as another exception than part of a sustained pattern. The disparity of technology and resources between the two sides meant that one had to fight a guerilla war, which leaves very little room for either combatants to be 'honourable' in their methods. Both essentially relied upon ambush for the majority of the war, using claymores and the like. The Viet Minh then used human shields by melting back into the general populace, and the US burning down forests with Napalm and carpet bombing surrounding countries to try and break the VC supply lines.
If you go back to the medieval era, just about every war was fought similar to Vietnam. Knights would ride out from their castles, burn enemy lands and then ride back for shelter. True, if a knight was captured, he could expect to be treated well and ransomed, but the common soldier had no such priveleges.
In short, there is no real 'honour' in warfare - if you read soldier's accounts of WW1, you'll soon realise how they felt about it, many of them explicitly denying any humanity in the conflict. What people don't realise is that it doesn't take technology to remove someone's human compassion once a war gets underway.
EDIT: Sorry for the s*** grammar, just woken up...
