noun
foolproof plan; infallible measure
A plan or measure that is perfectly safe and leaves no room for failure. Often used in formal or strategic contexts.
彼は万全の策を講じてプロジェクトに臨んだ。
He approached the project with a foolproof plan.
災害に備えて万全の策を取る必要がある。
We need to take infallible measures to prepare for disasters.