Validation is a particular issue with political-military simulations, since much of the data produced is subjective. One controversial doctrine that arose from early post-WWII simulations was that of "signalling"—the idea that by making certain moves, it is possible to send a message to your opponent about your intentions: for example, by conspicuously conducting field exercises near a disputed border, a nation indicates its readiness to respond to any hostile incursions. This was fine in theory, and formed the basis of east–west interaction for much of the cold war, but was also problematic and dogged by criticism. An instance of the doctrine's shortcomings can be seen in the bombing offensives conducted by the United States during the Vietnam War. US commanders decided, largely as a result of their "Sigma" simulations, to carry out a limited bombing campaign against selected industrial targets in North Vietnam. The intention was to signal to the North Vietnamese high command that, whilst the United States was clearly capable of destroying a much greater proportion of their infrastructure, this was in the nature of a warning to scale down involvement in the South 'or else'. Unfortunately, as an anonymous analyst said of the offensive (which failed in its political aims), "they either didn't understand, or did understand but didn't care". It was pointed out by critics that, since both Red and Blue teams in Sigma were played by Americans—with common language, training, thought processes and background—it was relatively easy for signals sent by one team to be understood by the other. Those signals, however, did not seem to translate well across the cultural divide.