  The pentagram map, introduced by Schwartz in 1992, is a dynamical system on
the moduli space of polygons in the projective plane. Its real and complex
dynamics have been explored in detail. We study the pentagram map over an
arbitrary algebraically closed field of characteristic not equal to 2. We prove
that the pentagram map on twisted polygons is a discrete integrable system, in
the sense of algebraic complete integrability: the pentagram map is birational
to a self-map of a family of abelian varieties. This generalizes Soloviev's
proof of complex integrability. In the course of the proof, we construct the
moduli space of twisted $n$-gons, derive formulas for the pentagram map, and
calculate the Lax representation by characteristic-independent methods.
