  We prove a converse to well-known results by E. Cartan and J. D. Moore. Let
$f\colon M^n_c\to\Q^{n+p}_{\tilde c}$ be an isometric immersion of a Riemannian
manifold with constant sectional curvature $c$ into a space form of curvature
$\tilde c$, and free of weak-umbilic points if $c>\tilde{c}$. We show that the
substantial codimension of $f$ is $p=n-1$ if, as shown by Cartan and Moore, the
first normal bundle possesses the lowest possible rank $n-1$. These
submanifolds are of a class that has been extensively studied due to their many
properties. For instance, they are holonomic and admit B\"{a}cklund and
Ribaucour transformations.
