  How fast must an oriented collection of extensile swimmers swim to escape the
instability of viscous active suspensions? We show that the answer lies in the
dimensionless combination $R=\rho v_0^2/2\sigma_a$, where $\rho$ is the
suspension mass density, $v_0$ the swim speed and $\sigma_a$ the active stress.
Linear stability analysis shows that for small $R$ disturbances grow at a rate
linear in their wavenumber $q$, and that the dominant instability mode involves
twist. The resulting steady state in our numerical studies is isotropic
hedgehog-defect turbulence. Past a first threshold $R$ of order unity we find a
slower growth rate, of $O(q^2)$; the numerically observed steady state is {\it
phase-turbulent}: noisy but {\it aligned} on average. We present numerical
evidence in three and two dimensions that this inertia driven flocking
transition is continuous, with a correlation length that grows on approaching
the transition. For much larger $R$ we find an aligned state linearly stable to
perturbations at all $q$. Our predictions should be testable in suspensions of
mesoscale swimmers [D Klotsa, Soft Matter \textbf{15}, 8946 (2019)].
