Cultural distance.
Consider the enemy as "inferior forms of life,". As simple is that. The propaganda machine of a country can prepare the future soldiers to look at the enemy like that. The book quotes War on the Mind by Peter Watson: "to get the men to think of the potential enemies they will have to face as inferior forms of life [with films] biased to present the enemy as less than human: the stupidity of local customs is ridiculed, local personalities are presented as evil demigods."
This of course greets us back at places like KC main where the narration of the posters often turns for such tools as namecalling which was very common on battlefields. Kraut, gook, Tommy or the simple Vietnam era "body count" mentality is all part of it this type of distance.
Using racial differences is a classic example, Grossman here pulls the Nazi card but at least mentions the colonial conflicts too.
It's easier to create this distance against someone who seems more alien in the first place 44 percent of American soldiers in World War II said they would "really like to kill a Japanese soldier," but only 6 percent expressed that degree of enthusiasm for killing Germans. In Vietnam the US made an effort not to play this card against the Vietnamese as their ally was the same as the enemy.
Sometimes even minor differences (for an outsider at least) can be magnified well out of proportion like it happened during the Yugo Wars.