  It is proved that the theta series of an even lattice whose level is a power
of a prime $\ell$ is congruent modulo $\ell$ to an elliptic modular form of
level~1. The proof uses arithmetic and algebraic properties of lattices rather
than methods from the theory of modular forms. The methods presented here may
therefore be especially pleasing to those working in the theory of quadratic
forms, and they admit generalizations to more general types of theta series as
they occur e.g. in the theory of Siegel or Hilbert modular forms.
