  The fundamental construction underlying descent theory, the lax descent
category, comes with a functor that forgets the descent data. We prove that, in
any $2$-category $\mathfrak{A} $ with lax descent objects, the forgetful
morphisms create all Kan extensions that are preserved by certain morphisms. As
a consequence, in the case $\mathfrak{A} = \mathsf{Cat} $, we get a monadicity
theorem which says that a right adjoint functor is monadic if and only if it
is, up to the composition with an equivalence, (naturally isomorphic to) a
functor that forgets descent data. In particular, within the classical context
of descent theory, we show that, in a fibred category, the forgetful functor
between the category of internal actions of a precategory $a$ and the category
of internal actions of the underlying discrete precategory is monadic if and
only if it has a left adjoint. More particularly, this shows that one of the
implications of the celebrated Benabou-Roubaud theorem does not depend on the
so called Beck-Chevalley condition. Namely, we prove that, in indexed
categories, whenever an effective descent morphism induces a right adjoint
functor, the induced functor is monadic.
