They could tell people, and I'm sure many did, but I don't doubt the information censoring capabilities of ONI. A serving soldier that opens his or her mouth about certain facts of the war would risk breaking breaking moral, undermining public confidence in the UNSC, causing mass panic and harming the UNSC's war effort through a diminished economy as a result of the disaster. The number of military personal that are privy to the war's reality would have to know just how little the UNSC can actually do to save its people, and just how important it is to the fight that people remain confident in the UNSC, therefore I don't see any incentive for them to speak about it. In a way, they would be traitors to the cause. Of course there may have been some who, for whatever reason, decided to say something but with the blanket of cyber security that lies over the UNSC from all the AI's they posses I don't doubt that sources would be quickly found and dealt with before it became widespread, with the perpetrator being discredited as a shell shocked and deluded individual locally. It also depends on the extent of their own exposure to the war and how much of a view point they can get of each situation. Assuming they survive a lost battle, they may be told that reinforcements are on their way and that the fight isn't over yet, etc. Victories would also be over-emphasised with major defeats being a footnote and minor defeats probably not being mentioned at all. Or they might be passed off as "tactical victories", or "heroic last stands that gave the enemy a horrific pyrrhic victory." The idea I always struggled to comprehend with this idea is how in the hell people didn't notice that holidays to Harvest were suddenly forbidden, and that the list of unavailable immigration and holiday destinations was growing bigger. If people never asked questions about that then maybe immigration and other movement of people between colonies was uncommon under the UNSC. Or maybe a complete halt to immigration and such was justified on the basis that ships and resources were all required for the war effort and such, and/or that the likelihood of attacks from Covenant raids were too high for non-essential space travel. Hell, maybe they even created the impression that many colonies were still there and faked news broadcasts and news stories and activity from them and such. Though not all of them I'd imagine. The occasional news of a colony attacked would galvanize people into the fight.