  Recent numerical and observational studies contain conflicting reports on the
spectrum of magnetohydrodynamic turbulence. In an attempt to clarify the issue
we investigate anisotropic incompressible magnetohydrodynamic turbulence with a
strong guide field $B_0$. We perform numerical simulations of the reduced MHD
equations in a special setting that allows us to elucidate the transition
between weak and strong turbulent regimes. Denote $k_{\|}$, $k_\perp$
characteristic field-parallel and field-perpendicular wavenumbers of the
fluctuations, and $b_{\lambda}$ the fluctuating field at the scale $\lambda\sim
1/k_{\perp}$. We find that when the critical balance condition, $k_{\|}B_0\sim
k_{\perp} b_{\lambda}$, is satisfied, the turbulence is strong, and the energy
spectrum is $E(k_{\perp})\propto k^{-3/2}_{\perp}$. As the $k_{\|}$ width of
the spectrum increases, the turbulence rapidly becomes weaker, and in the limit
$k_{\|}B_0\gg k_{\perp} b_{\lambda}$, the spectrum approaches
$E(k_{\perp})\propto k_{\perp}^{-2}$. The observed sensitivity of the spectrum
to the balance of linear and nonlinear interactions may explain the conflicting
numerical and observational findings where this balance condition is not well
controlled.
