A common metaphor for this is to say you've been " thrown in the deep end ", referring to the (supposed) practice of teaching someone to swim by throwing them into a swimming pool at the deep end, where they'll drown if they don't figure out on their own how to swim.
ell.stackexchange.com/users/32/StoneyB on hiatus
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 .
ell.stackexchange.com/users/29924/Alan Carmack
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.
ell.stackexchange.com/users/41431/Patrick McElhaney
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” .
"Coffee break's over.
Back on your heads.
" More relevant than it may sound :-).
Joke's a bit long to explain well, so many of these explain it or a short text version here .
In this context - you get into a new situation and THINK you know what its all about, until the door opens and ....
Oh No !!! :-)
ell.stackexchange.com/users/21556/Russell McMahon
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.
ell.stackexchange.com/users/41426/FacticiusVir
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...
Assigned to a job I know nothing about it - is there an idiom for that idea?
ell.stackexchange.com
<idioms><idiom-request>
ell.stackexchange.com/questions/102957
ell.stackexchange.com/users/41408/dmoya
Thrown in the deep end, baptism by fire, thrown to the wolves, being up to your neck in alligators, sink or swim, sheep among wolves, or up shit creek are all suggestions in this context. Metaphors for this are "thrown in the deep end", "baptism by fire", "thrown to the wolves", "Up to his (or your) neck in alligators" (or other) and "Sink or swim". If the environment is hostile due to your newness,  "Sheep among wolves" would work. "Shit creek" also conveys the meaning, albeit more crudely.
A metaphor for this is "thrown in the deep end". Another idiom is "baptism by fire". A similar idiom is "thrown to the wolves". Another phrase for this is "Up to his (or your) neck in alligators" (or other). If the environment is hostile due to your newness,  "Sheep among wolves" would work. "Sink or swim" could be used as well. "Shit creek" also conveys the meaning, albeit more crudely.