  We determine necessary and sufficient conditions under which a large class of
relativistic generalizations of Braginskii's magnetohydrodynamics, described
using Israel-Stewart theory, are causal and strongly hyperbolic in the fully
nonlinear regime in curved spacetime. Our new nonlinear analysis provides
stricter constraints on the dynamical variables that cannot be obtained via a
standard linear expansion around equilibrium. Causality severely constrains the
size of shear-viscous corrections, placing a bound on the far-from-equilibrium
dynamics of magnetized weakly collisional relativistic plasmas, which rules out
the onset of the firehose instability in such systems.
