I constantly see people saying, unconditionally, that oh liberals are a lost cause, neo-Nazis are a lost cause, police are a lost cause, soldiers are a lost cause. And often, yes, it's not worth the effort to engage them and doubly so if you're a single regular unskilled outsider.
However, those assumptions are factually false. There are many well-known examples of people from all those groups becoming antifascists, communists and anarchists. It's obviously a transition which can happen. The question becomes, can it be done reliably enough that it becomes worthwhile to target these groups? That will inevitably depend on current conditions, there's no abstract ultimate answer.
OP made it clear that the military are one of the high-value groups. Every military member we convert is a skilled insider (an espionage dream come true) who won't try to kill our comrades later, one more asset detracting from the imperial machine, and one more who won't get picked up by neo-fascists who want to harass us. The armies are filled with people economically oppressed by the state, only joining for mediocre benefits. I believe if there is an effective way to radicalize military members to socialism, it's a very worthwhile endeavour. Of course, that's a very big 'if'!
>>434 mentioned Eyes Left which is useful for understanding this question. They openly share the perspective of anti-capitalist ex-military personnel, interview people to discover their paths away from the military and towards radicalization, what help they received on their journey (civilian therapists are a noteworthy theme) and other great information. Plus, they're in-network, they can talk to other veterans with sympathy. Other 'unpopular wars' should be a useful case study, like understanding how soldiers in the Vietnam War were able to foster such discontent in the military (e.g. circulating zines, radio).