Most of the posts I've seen concerning the most useful knives in a combat theater boil down to those that have "been there" recommending a decent-quality folder (but not too expensive, because it will probably get lost or stolen) and a good multi-tool.
The consensus is usually that big knives will either not be allowed by your commander, or will be mostly useless and just add extra weight for you to lug around. If you just must carry a fixed blade, pick something small and tough. Aim for utility rather than a "fighting knife" as the chances of you needing the latter are vanishingly small.