Whoop, here it is...

http://malditoscyborgs.org/dam/english/articles/conpiracy/esp_and_mind_control.html

"The Vietnam war, with its exceedingly stressful combat environment of no clear-cut enemies combined with ambush being the normal way of making contact, produced a large body of individual reports stressing enhanced perception and awareness."

Here, I beleive, we have an answer...