Maybe a lot depends on the Rules of Engagement as suggested earlier. The affairs in Vietnam, Afghanistan and Iraq are all examples in which the high-tech force is hampered by it being a 'low key' conflict, media-consciousness, unpopularity back home, etc, etc.
Consider instead the German occupation of Europe. Those guys weren't worried about TV cameras or the 'hearts and minds' of the locals.
They did a pretty good job of maintaining control even though there wasn't a great TL disparity between themselves and the Resistance. Every time a soldier got killed by an 'insurgent', ten townsfolk were lined up against the wall. Seemed to work for the Nazis.
If the guy with the battledress has both hands tied behind his back with red tape, of course he'll have a hard time controlling the natives. Occupation policy and rules of engagement are paramount IMO.
I think that it's important not to overstate the "lessons" of Vietnam. Vietnam was actually a very unusual insurgency. And query whether "victory" is an accurate term for a conflict in which the "victors"
admit to losing 20 dead for every enemy killed.
Anyhow, the US failed in Vietnam (IMHO) due to a perfect storm of circumstances that are unlikely to appear regularly in the future--especially in the Imperium. In order of importance, here's my list:
1. The insurgents were supplied by the Chinese and Soviets. Their source of supply was immune from US attack. As far as I know, few insurgencies have failed when this was the case. See the Soviet failure in Afghanistan for supporting evidence and the British success in Malaysia (where the insurgents
were effectively isolated from their sources of supply). The Imperium has the ability to completely isolate systems from resupply by unfriendly governments, so they won't typically face this situation.
2. President Johnson made a cynical and unprecedented decision
not to deploy the force necessary to win, and then decided not to withdraw. This guaranteed a strategic stalemate. Johnson's subsequent humiliation serves as an effective warning for future politicians. And in any case, Johnson's calculations were motivated by the desire to convince the voters that his social welfare policies were more important than some distant war. An autocratic regime like the Imperium would not need to make such calculations.
3. Predictably, the US people tired of that stalemate and questioned whether the cost was worth the purported benefit. This will, IMHO, inevitably happen in a democracy, so #2 should be avoided at all costs. Citizens of autocracies will have far less ability to influence policy like this.
4. The US media managed to convert the Tet Offensive -- a decisive Viet Cong defeat (even they thought so) -- into a US defeat. They were unable to repeat this feat with Iraq, which leads me to conclude that this was a one-time occurence (though I predict they'll keep trying). I think that the degree of media culpability in the US defeat has been overstated (although I think it's highly probative of the systemic anti-US bias in the US media). However, the media has been
highly effective in pushing the false "Vietnam was doomed to fail" meme. The war, like most wars, was winnable. The US simply failed to do what was necessary to do so. The influence of the media is likely to be far less in the Imperium for the simple reason that communication is limited to the speed of travel.
So I'd caution Traveller referees from using Vietnam -- especially the ridiculous media version of Vietnam in movies like
Platoon,
Apolcalypse Now, etc., as a model for future insurgencies. Malaysia, Iraq and Afghanistan are far more relevant to what insurgencies would like in the Imperium. And in particular, don't overrate the ability of the media to lose insurgencies. They've really only done it once (and had lots of help).