under necessity

But he was under necessity to assist in its government and defense, as well as to act as its physician.

I was under necessity, as you shall hear, to visit Chapelizod.

They feared no men even of "twice their own heavy and big," but would never fight except under necessity.

Never, save under necessity, should the parent yield up his sacred duty to another, at least during the tender years of childhood.

Under necessity only will beavers gnaw dead wood, and then only to a limited extent.