‎The elite of the marine corps is a group called Force Reconnaissance. These troops are employed in special operations both “green” (in which having had to engage the enemy is deemed as failure.

What does the “ in which having had to engage the enemy” mean here? And what grammar is used?

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This can be a confusing grammatical structure. It's a past participle (having) and a participle (had). It doesn't have a specific name, but an example is "Having had eaten, he decided not to go to the restaurant." It means something like, "Because he had eaten, he decided not to go.."

It's, essentially, mixing past and present tenses while replacing a repetitive clause. So the passage means "in which if they had had to engage the enemy..." (so, if they had to fight with the enemy, the mission was a failure). When 'engage' is used in the military, it means to fight with.