TOKYO—Prime Minister Shinzo Abe declared a nationwide state of emergency and more cash payouts to shield the blow, imposing more economy-stifling measures in a country that had mostly avoided them before.

The declaration, coming after rising criticism, amounted to an acknowledgment that his efforts to keep Japan running normally had exposed the country to a potential sharp rise in coronavirus infections. That is a potential lesson for the U.S. and European countries that are weighing when and how to resume everyday activities.

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