When the objective of an army is killing as an end in itself, rather than as the means to the end of advancing geographically, any moral bearings are at risk, to say the least. That happened to the armed forces of the U.S. in Vietnam. It is fundamentally the nature of ideological/theological conflict, as opposed to the paradigm of invasion/defense of geopolitical space (e.g. the Russia/Ukraine conflict)--which is morally precarious enough--and certainly applies to all participants in such conflicts.