TOKYO >> As tensions rise on the Korean Peninsula, one thing remains certain: All sides have good reason to avoid an all-out war. The last one, six decades ago, killed an estimated 4 million people.

A war would be suicidal for North Korea, which cannot expect to defeat the United States and successfully overrun South Korea. War would be horrific for the other side as well. South Korea could suffer staggering casualties. The U.S. would face a destabilized major ally, possible but unlikely nuclear or chemical weapons attacks on its forward-positioned bases, and dramatically increased tensions with North Korea’s neighbor and Korean War ally, China.

Here’s a look at the precarious balance of power that has kept the Korean Peninsula so close to conflict since the three-year war ended in 1953, and some of the strategic calculus behind why, despite the shrill rhetoric and seemingly reckless saber-rattling, leaders on both sides of the Demilitarized Zone have carefully avoided going back over the brink.

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THE SEA OF FIRE

Even without nuclear weapons, North Korea has an ace in the hole. Most experts believe its claims to have enough conventional firepower from its artillery units to devastate the greater Seoul area, South Korea’s bustling capital of 24 million. Such an attack would cause severe casualties — often estimated in the hundreds of thousands — in a very short period of time.

Many of these artillery batteries are already in place, dug in and very effectively camouflaged, which means that U.S. and South Korean forces cannot count on being able to take them out before they strike. Experts believe about 60 percent of North Korea’s military assets are positioned relatively close to the Demilitarized Zone separating the countries.

North Korea’s most threatening weapons are its 170 mm Koksan artillery guns, which are 14 meters long and can shoot conventional mortar ammunition 40 kilometers (25 miles). That’s not quite enough to reach Seoul, which is 50 kilometers (30 miles) from the DMZ. But if they use rocket-assisted projectiles, the range increases to about 60 kilometers (37 miles). Chemical weapons fired from these guns could cause even greater mayhem.

North Korea experts Victor Cha and David Kang posted on the website of Foreign Policy magazine late last month that the North can fire 500,000 rounds of artillery on Seoul in the first hour of a conflict.

Even so, not everyone believes North Korea could make good on its “sea of fire” threats. Security expert Roger Cavazos, a former U.S. Army officer, wrote in a report for the Nautilus Institute last year that, among other things, North Korea’s big guns have a high rate of firing duds, pose more of a threat to Seoul’s less populated outer suburbs, and would be vulnerable to counterattack as soon as they start firing and reveal their location.

“North Korea occasionally threatens to “turn Seoul into a Sea of Fire,” he wrote. “But can North Korea really do this? … The short answer is they can’t; but they can kill many tens of thousands of people, start a larger war and cause a tremendous amount of damage before ultimately losing their regime.”

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FIRST STRIKES, PRE-EMPTIVE STRIKES

This is what both sides say concerns them the most.

North Korea says it is developing nuclear weapons and long-range missiles as a deterrent to keep the United States or South Korea from attacking it first. The reasoning is that Washington will not launch a pre-emptive strike if North Korea has a good chance of getting off an immediate — and devastating — response of its own.

Along with its artillery aimed at Seoul and other targets in South Korea, North Korea is developing the capacity to deploy missiles that are mobile, thus easier to move or hide. North Korea already has Rodong missiles that have — on paper at least — a range of about 1,300 kilometers (800 miles), enough to reach several U.S. military bases in Japan. Along with 28,000 troops in South Korea, the U.S. has 50,000 troops based in Japan.

North Korea is not believed to be capable of making a nuclear weapon small enough to fit on a long-range missile capable of hitting the United States. But physicist David Albright, president of the Institute for Science and International Security, believes it may be capable of mounting nuclear warheads on Rodongs. In any case, Pyongyang is continuing to pursue advancements, apparently out of the belief that it needs nuclear-tipped missiles capable of reaching the U.S. to have a credible deterrent.

The United States rejects the North’s claim that such a deterrent is necessary, saying it does not intend to launch pre-emptive strikes against North Korea. At the same time, Washington has made it clear that it could.

During ongoing Foal Eagle military maneuvers in South Korea, two U.S. B-2 strategic stealth bombers, flying from their base in Missouri, conducted a mock bombing run on a South Korean range. The B-2 is capable of carrying nuclear weapons, precision bombs that could take out specific targets such as North Korean government buildings, and massive conventional bombs designed to penetrate deep into the ground to destroy North Korean tunnels and dug-in military positions. One big problem, however, is determining where the targets are.

Amid heightened tensions over North Korea’s nuclear weapons program in 1994, President Bill Clinton reportedly considered a pre-emptive strike, but decided the risks were too high.

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CHINA’S DILEMMA

Without China, North Korea wouldn’t exist. The Chinese fought alongside the North Koreans in the Korean War and have propped up Pyongyang with economic aid ever since.

Beijing has grown frustrated with Pyongyang, especially over its nuclear program. China and the U.S. worked together in drafting a U.N. resolution punishing the North for its Feb. 12 nuclear test.

But China still has valid reasons not to want the regime to suddenly collapse.

War in Korea would likely spark a massive exodus of North Korean civilians along its porous 1,300-kilometer (800-mile) border, which in turn could lead to a humanitarian crisis or unrest that the Chinese government would have to deal with. The fall of North Korea could pave the way for the United States to establish military bases closer to Chinese territory, or the creation of a unified Korea over which Beijing might have less influence.

China, the world’s second-largest economy, also has significant trade with South Korea and the United States. Turmoil on the Korean Peninsula would harm the economies of all three countries.

Patrick Cronin, an Asia expert at the Center for a New American Security and a senior State Department official during the George W. Bush administration, said Beijing is helping set up back-channel negotiations with North Korea to ease the tensions. But he warned that the U.S. isn’t likely to win China over as a reliable partner against North Korea beyond the current flare-up.

“There are limits to how far China and the U.S. have coincidental interests with regard to North Korea,” he said.