  Identifying an entanglement island requires exquisite control over the
entropy of quantum fields, which is available only in toy models. Here we
present a set of sufficient conditions that guarantee the existence of an
island and place an upper bound on the entropy computed by the island rule.
This is enough to derive the main features of the Page curve for an evaporating
black hole in any spacetime dimension. Our argument makes use of Wall's maximin
formulation and the Quantum Focusing Conjecture. As a corollary, we derive a
novel entropy bound.
