I'd like to know an idiom that let me express the idea of being assigned to a job/task you have no clue at all. My first language is Spanish, and we have a cool idiom for this situation (rough translation: I was sent straight to war!), but I don't know if there is a similar one in English...
Another idiom is baptism by fire:
A phrase originating from Europe that describes an employee that is learning something the hard way, like being immersed in their field of employment. Baptism by fire has its roots in battle terminology, describing a soldier's first time in battle.
Thus this usage seems to match the subject matter of your own military idiom.
Read more: Baptism by Fire (Investopedia).
Note that this military usage ultimately originates in the concept baptism by fire found in the Christian Gospels. See Wikipedia.
A similar idiom is thrown to the wolves, which is what we say when we assign someone to a job with very little assistance or training. The "wolves" in this case are people who depend on the job being done, and are accustomed to it being done up to a certain standard. Much like a person who's been sent to war, a person who has been thrown to the wolves needs to learn to survive in a hostile environment. They'll either learn very quickly or fail miserably.
Related to this is the Peter Principle-
The Peter principle is a concept in management theory formulated by Laurence J. Peter and published in 1969. The theory is that the selection of a candidate for a position is based on the candidate's performance in their current role, rather than on abilities relevant to the intended role. Thus, employees only stop being promoted once they can no longer perform effectively, and managers rise to the level of their incompetence.
I saw the question in the sidebar and came, a day too late, with "thrown in at the deep end". A few relevant expressions came to mind. After I think of each one I searched to see if it was here and in more cases than not, some variant had already been suggested. Where my versions seem usefully different I've still included them, with comment on their "duplicity" :-)
- "Out of his depth" — similar but not identical in concept with "thrown in ..."
Related if not quite apposite:
Up to his (or your) neck in alligators" (or other).
Related related: "When you're up to your neck in alligators it's hard to remember that you came to clear the swamp.
More if the crowd you are placed with are dangerous to you because of your innocence or newness:
"Sheep among wolves".
Conveniently "sheep" can be considered singular or plural there, or you can add a leading "A " for singular. (Some similarity with Patrick's “thrown to the wolves.”)
I thought of "sink or swim" but I see Nigel also did 21 hours before me — in his comments to StoneyB
"Up the creek without a paddle" - a more common form of what I see user3169 commented yesterday. A not uncommon somewhat crude variant is the “shit creek”.
In this context - you get into a new situation and THINK you know what its all about, until the door opens and .... Oh No !!! :-)