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_ {\parallel}, 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_ {\parallel} 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_ {\parallel} width of the spectrum increases, the turbulence rapidly becomes weaker, and in the limit k_ {\parallel} B_ {0}\gg k_ {\perp} b_ {\lambda}, the spectrum approaches E (k_ {\perp})\propto k^{-2} _ {\perp}. 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.