in time of need; should an emergency arise
A formal, somewhat literary set phrase used to refer to a hypothetical future crisis or emergency. Often appears in discussions of preparedness, national defense, or contingency planning.
In the event of an emergency, measures to protect the safety of the people are required.
He is always prepared for any contingency.
緊急時 is a more common, neutral term for 'in an emergency,' while 一朝有事 is a formal, literary set phrase with a nuance of a sudden, unforeseen crisis.
A yojijukugo (four-character compound) combining 一朝 (one morning, suddenly) and 有事 (emergency, contingency). The phrase literally means 'one morning, an incident occurs,' conveying the idea of a sudden crisis.