A classical/formal variant of '〜ざるを得ない' used after the imperfective stem of a verb, meaning 'have no choice but to ~; be compelled to ~.'
雨が激しくなったため、試合は延期せざるを得ぬ。
Because the rain intensified, the match had to be postponed.
彼の誤りを認めざるを得ぬ状況になった。
He found himself in a situation where he had to admit his mistake.
我々は計画を変更せざるを得ぬと判断した。
We decided that we had no choice but to change the plan.