SEOUL, South Korea — As the United States debates the wisdom of military action against North Korea, its allies in South Korea have largely moved on and reached an uncomfortable conclusion — that they may have no choice but to live with a nuclear-armed neighbor.

American and South Korean forces began twice-yearly war games on Monday aimed at preparing for a possible attack by the North. But the idea of trying to knock out the North’s nuclear arsenal with a pre-emptive strike is a nonstarter across the political spectrum in South Korea, where millions live in range of North Korean artillery and rockets.

“In South Korea, whether you are a conservative or progressive, military action is not an option you can choose,” said Koh Yu-hwan, a professor of North Korean studies at Dongguk University in Seoul, the capital. “It’s an option of mass destruction.”

That broad consensus was behind President Moon Jae-in’s assurances to South Korea last week, saying that he was firmly against a military strike on the North and that the Trump administration would seek the South’s consent before any such action. On Monday, Mr. Moon emphasized that the joint exercises this week were defensive in nature.