  We study the monotone nonlinear energy stability of
\textit{magnetohydrodynamics plane shear flows, Couette and Hartmann flows}. We
prove that the least stabilizing perturbations, in the energy norm, are the
two-dimensional spanwise perturbations and give some criti\-cal Reynolds
numbers Re$_E$ for some selected Prandtl and Hartmann numbers. This result
solves a conjecture given in a recent paper by Falsaperla et al.
\cite{FMP.2022} and implies a Squire theorem for nonlinear energy: the less
stabilizing perturbations in the \textit{energy norm} are the two-dimensional
spanwise perturbations. Moreover, for Reynolds numbers less than Re$_E $ there
can be no transient energy growth.
