At approximately 12:29 p.m. Sunday in Punggye-ri, North Korea, a 6.3-magnitude earthquake was detected — it reportedly marked the country's sixth nuclear test and by far its largest yet. More detailed analysis will follow in the days ahead, including whether the North's claim of having successfully detonated a hydrogen warhead is credible.

This is hardly the moment to show daylight between Seoul and Washington, as President Donald Trump appears to be doing.

Before Sunday's test, news reports spoke of Trump's impending decision to withdraw from the U.S.-South Korea free-trade agreement (KORUS). Such a move is unjustified in the first place. But after Pyongyang's most threatening nuclear test so far, withdrawing from KORUS would send the worst possible signal to North Korea and China. If the White House proceeds to drop KORUS even after the test, it will severely damage one of America's staunchest and important allies in Asia by throwing the U.S.-South Korea alliance to the wolves.

Trump and South Korean President Moon Jae-in ought to respond to the regime of Kim Jong Un jointly and without any ambiguity. For Trump, the temptation to hint about a preventive strike against North Korea will surely grow. Some members of Congress may advocate for such a blow to end North Korea's nuclear threat once and for all.

But a preventive strike is the worst possible option for South Korea.

Any preventive strike on North Korean nuclear sites will result in North Korean attacks on U.S. bases in South Korea, as well as on civilian targets. Even if a small-yield nuclear weapon were dropped in Seoul, hundreds of thousands would die, and millions would suffer from radiation poisoning. South Korea as we know it, the world's 12th-largest economy, the sixth-largest trading power and Asia's most vibrant democracy, would cease to exist.

Trump must be prepared for a full-fledged war if he decides on a U.S. first strike.

Trump has every right to protect the welfare and safety of American citizens. But so, too, does Moon for 50 million South Koreans. Moon is entitled to disagree with the White House on a preventive attack.

For his part, Moon must end all engagement with North Korea. Kim rebuffed his proposal to jump-start inter-Korean dialogue. Moon wants to reopen the shared Kaesong Industrial Complex and resume tourist trips if South-North relations improve. But a North Korea that has hydrogen bombs is not a partner.

Moon must reject advisers who advocate extensive engagement with the North, provision of massive humanitarian assistance, or the toning down of U.S.-South Korea military exercises. No one is against dialogue, but the Moon administration's one-sided love affair with Pyongyang must end.

For now, the U.S., South Korea and Japan must sing from the same page. This means going through with the rest of the Terminal High Altitude Air Defense (THAAD) missile-defense deployments in South Korea and accelerating deterrence and defense assets.

Many in Seoul have talked about the reintroduction of U.S. tactical nuclear weapons into South Korea. That's one option. South Korea's offensive capabilities against North Korea can no longer be stymied by tacit pressures from Washington. That doesn't mean South Korea should pursue its own nuclear deterrent as Britain, France and Israel have done. But South Korea cannot rely solely on U.S. deterrence; it has to accelerate the development and deployment of its own offensive weapons that can hit all North Korean targets.

If there's a need to tamper with KORUS, the goal should be renegotiation, not unilateral U.S. abrogation. It's true that United States has a $28 billion trade deficit in goods and services with South Korea, mostly in automobiles and electronics. But imports of American cars have risen since KORUS was signed — and the U.S. service surplus of $6.9 billion in 2011 rose to $10.7 billion in 2016.

In 2016, South Korea imported $591 million in arms from the United States, or nearly 38 percent of its total arms imports. South Korea also provides $831 million for defense cost-sharing and assumed $767 million of the cost of building a U.S. military base in Pyeongtaek, the largest in Asia.

This is no time for a trade dispute between Seoul and Washington. No ally has been as loyal as South Korea. We need a united front; there is no other option.

Chung Min Lee is a professor of International Relations at the Graduate School of International Studies at Yonsei University in Seoul.