  We use first-principles theory to predict that the application of uniaxial
compressive strain leads to a transition from an antiferromagnetic insulator to
a ferromagnetic half-metal phase in LaMnO$_3$. We identify the Q2 Jahn-Teller
mode as the primary mechanism that drives the transition, indicating that this
mode can be used to tune the lattice, charge, and spin coupling. Applying
$\simeq$ 6 GPa of uniaxial pressure along the [010] direction activates the
transition to a half-metallic $\textit{pseudo-cubic}$ state. The
half-metallicity opens the possibility of producing colossal magnetoresistance
in the stoichiometric LaMnO$_3$ compound at significantly lower pressure
compared to recently observed investigations using hydrostatic pressure.
