  We examine the nonlinear classical dynamics of a fundamental string in
anti-de Sitter spacetime. The string is dual to the flux tube between an
external quark-antiquark pair in $\mathcal{N}=4$ super Yang-Mills theory. We
perturb the string by shaking the endpoints and compute its time evolution
numerically. We find that with sufficiently strong perturbations the string
continues extending and plunges into the Poincar\'e horizon. In the evolution,
effective horizons are also dynamically created on the string worldsheet. The
quark and antiquark are thus causally disconnected, and the string transitions
to two straight strings. The forces acting on the endpoints vanish with a power
law whose slope depends on the perturbations. The condition for this transition
to occur is that energy injection exceeds the static energy between the
quark-antiquark pair.
