Good English translation for French military term "Baptême du feu"

Solution 1:

The expression is used in English also:

• Phrase baptism of fire "a soldier's first experience of battle" (1857) translates French baptême de feu.

(Etymonline)

Baptism of fire:

• A severe ordeal or test, especially an initial one, as in This audition would be Robert's baptism of fire. This term transfers the original religious rite of baptism, whereby holiness is imparted, to various kinds of ordeal. At first it signified the death of martyrs at the stake, and in 19th-century France it was used for a soldier's first experience of combat.

(AHD)

also baptism by fire:

• In the military usage, a baptism by fire refers to a soldier's first time in battle. The Catholic Encyclopedia, and writers such as John Deedy, state that the term in a military sense entered the English language in 1822 as a translation of the French phrase baptême du feu. From military usage the term has extended into many other areas in relation to an initiation into a new role - for example the directorship of an arts festival.

(Wikipedia)

Note that the common current meaning of the expression is:

Baptism of/by fire ​

a very difficult first experience of something: - I was given a million-dollar project to manage in my first month - it was a real baptism of fire.

(Cambridge Dictionary)

Currently it is used more loosely for any difficult first encounter.

(AHD)