The United States and the Republic of South Korea have, until now, had identical interests in the Korean peninsula: defending against a North Korean attack on the South, and keeping the North’s regime at bay until it collapsed from internal contradictions.

The inevitable ability of North Korea to hit North America with a nuclear-armed intercontinental ballistic missile (ICBM) means the U.S. has to consider striking North Korea preventively, regardless of the casualties in South Korea because no U.S. President will trade San Francisco for Seoul.

Sen. Lindsey Graham Lindsey Olin GrahamMomentum growing among Republicans for Supreme Court vote before Election Day Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Warning signs flash for Lindsey Graham in South Carolina MORE (R-S.C.) recently said President Trump told him, "If there’s going to be a war to stop [Kim Jong Un], it will be over there. If thousands die, they’re going to die over there. They’re not going to die here.” And National security adviser H.R. McMaster has stated the U.S. is planning a “preventive war” against North Korea.

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What will the president have to consider before he launches a preventative attack on North Korea?

Casualties in an attack of the North on the South are estimated at 100,000 in Seoul in the first 24 hours. The U.S. military estimates 200,000-300,000 South Korean and U.S. military casualties within 90 days, and even more civilian deaths, many of which may be caused not by North Korean weapons but the collapse of the electric power grid, and the water, transport and sewer systems in a city with one of the highest population densities in the world. Half of the South’s population of over 50 million lives in the Seoul Capital Area, which produces almost half of the country’s gross domestic product.

The effects would be felt worldwide and immediately as South Korea is a vital part of the global supply chain for high technology equipment, both as end products and parts used by other manufactures. Nor is it likely companies in other countries can quickly pick up the slack: it is estimated that the replacement cost of the display manufacturing capability of Samsung and rival LG will top $50 billion. In the words of one analyst, “If Korea is hit by a missile, all electronics production will stop.”

Shipping in the nearby Sea of Japan, East China Sea, and Yellow Sea will halt as there may no longer be a destination for the cargo, and spiking maritime insurance rates, if insurance can be had, will make most voyages unprofitable. Shipping to and from major Chinese ports such as Dalian, Qingdao, Shanghai, and Tianjin will halt and disrupt worldwide supply chains. Ships returning to China will have to anchor until the crisis abates, at a cost to the shipping lines (and customers). Most of Japan’s major ports are on the east coast of the main island, Honshu, and will be open for business, though with the threat of North Korean missiles early in the conflict.

South Korea imports 98 percent of its fossil fuels and relies exclusively on tankers for liquefied natural gas (LNG) and crude oil. China will also be affected as it is the world’s largest net importer of crude oil and has LNG regasification terminals at Dalian, Qingdao, Shanghai, and Tianjin. Crude and LNG tankers enroute will have to be rerouted, but the product can probably be sold on the spot market.

The airspace surrounding the Korea Peninsula and northeast China will be closed and will affect passenger and cargo traffic, including at Beijing, the world’s second busiest airport, and Shanghai, the ninth busiest. Eastward traffic to the region will slow and will hit the hub airport, Dubai, which is also a major tourist destination for Asia. Japan will lose eastbound air traffic, but westward traffic from the U.S. less so.

South Korea imports most of its food as it has little arable land. The U.S. is its largest supplier, providing mostly corn, meat, hides, soybeans, milling wheat, and cotton, so the U.S. farm sector will sag if the crisis happens when produce is on the way to market.

Of the local allies, Japan may be more disposed to action as it isn’t – literally – on the front line and it has already deployed the PATRIOT surface-to-air interceptor and the AEGIS ship-based anti-ballistic missile system, and it may install the AEGIS Ashore system or the Terminal High Altitude Area Defense (THAAD) system. South Korea will be more reticent as it will absorb the initial blows and the only air defense missiles on its territory, the THAAD system, were deployed by the U.S. in the spring of 2017.

President Trump will have to weigh Asia’s regional stability and homeland security when making the toughest call since President Truman OK’d the use of nuclear weapons in Japan in 1945.