(CNN)Things have been tense on the Korean Peninsula and many North Korea watchers believe the situation is dangerous.
Why neither North Korea nor the United States want all-out war
But it hasn't reached the brink yet, and that's likely because US President Donald Trump and North Korean leader Kim Jong Un and their respective advisers are aware of the immense cost of the Korean War, which started June 25, 1950.
"If this goes to a military solution, it is going to be tragic on an unbelievable scale," Defense Secretary James Mattis said at a news conference in May.
The Korean War, which technically never ended, led to the deaths of about 600,000 North Korean and 1 million South Korean civilians, along with hundreds of thousands of troops.
The legacy of the war lives on in North Korea, where it's still used as a key piece of propaganda for the Kim regime. American fighter jets blanketed the country with about 625,000 tons of bombs on North Korea, killing 20% of the country's population, according to one estimate.
If the conflict reignited, it could be even more cataclysmic this time around with the specter of nuclear weapons looming.
"The threat of war on the peninsula -- major war or a limited war -- has been present off and on since the end of the Korean War," Adam Mount, a senior fellow at the Center for American Progress, told CNN. "Both the United States historically and also North Korea have proved that they can demonstrate restraint, de-escalate a crisis (and) step back from the brink of war ... They're both interested in avoiding a war that would be in nobody's interest."
While the tense situation on the Korean Peninsula could escalate quickly, there are a handful of scenarios that could play out, ranging from something as calamitous as nuclear strike to small-scale artillery attacks that do not devolve into all-out war, which happened in 2010.
"Part of the difficulty of discussing something like this is there's a wide range of possible contingencies that vary widely in terms of the outlook in terms of how damaging they are," Mount said. "The challenge is to try to control escalation."
The biggest danger is in densely-populated cities, places like Seoul (urban area population around 9.7 million) or Tokyo (urban area population around 38 million).
Nuclear strikes on those capitals would be catastrophic in terms of loss of life, but an attack using conventional weapons could kill tens of thousands of people too.
"Combat in another Korean War would take place in Seoul's crowded suburbs. While our war planners estimated that US and South Korean forces would contain the North Korean advance north of Seoul, the price of defense would be heavy," Ash Carter and William Perry, two former US defense secretaries, wrote in a 2002 op-ed in the Washington Post.
"Thousands of US troops and tens of thousands of South Korean troops would be killed, and millions of refugees would crowd the highways. North Korean losses would be even higher. The intensity of combat would be greater than any the world has witnessed since the last Korean War," the op-ed said.
The concerns about a costly battle in Seoul remain today.
Roger Cavazos of the Nautilus Institute found in a 2012 study that there would be about 3,000 casualties in the first few minutes of a conventional North Korean artillery barrage focused on South Korean forces and 64,000 in the first day.
A surprise volley fired indiscriminately could kill nearly 30,000 civilians, the study found.
North Korea could also fire its ballistic missiles at US, South Korean and Japanese forces (or civilian populations) throughout the region.
North Korea is mismatched when it comes to military technology and capability, experts say. And it's believed that many of its armaments could be out of date or do not function properly, according to a 2016 five-part analysis on the North Korea threat from the geopolitical intelligence firm Stratfor.
Though North Korea's air force and navy pale in comparison to those of the United States, both pose a threat, according to Stratfor.
"Even without the nuclear threat, attacking North Korea guarantees massive destruction in return," the Stratfor analysis said.
If full-scale war broke out, a primary concern for the US would be to eliminate North Korea's nuclear threat.
The US Air Force might use 24 F-22 tactical fighters -- which can carry two 450-kilogram (1,000 pounds) bombs -- and 10 B-2 bombers -- which can deploy massive bombs to reach underground bunkers -- to eliminate North Korea's nuclear infrastructure, according to Stratfor.
"Because of their unique properties, these expensive, stealthy platforms would form the backbone of any anti-nuclear operations," it said.
The United States would also rely on its Tomahawk cruise missiles, incredibly precise weapons that can be fired from sea hundreds of miles away from their target, though collateral damage would likely also be an issue.
"The US Navy (with enough time to prepare) can surreptitiously park two of its four Ohio-class cruise missile submarines off the North Korean coast," according to Stratfor. "When combined with destroyers and cruisers from the 7th Fleet already in the area, the United States could use more than 600 cruise missiles for the mission."
A crucial problem is intelligence -- it's not clear exactly how many nuclear weapons the reclusive communist state has or precisely where they are.
The United States has a tough time gathering that sort of information on North Korea; former CIA Director Michael Hayden has called North Korea the toughest intelligence target on the planet.
Without this information, the US could fail to completely eliminate North Korea's rapidly progressing nuclear threat in the first throes of a war, potentially unable to stop an attack on the US mainland.
"Before the end of President Trump's current term, the North Koreans will probably be able to reach Seattle with an indigenously produced nuclear weapon aboard an indigenously produced intercontinental ballistic missile," said Hayden.
For South Korea, cost is an important factor.
A 2014 estimate from the South Korean National Assembly Budget Office found that reunification could cost as much as $9.2 trillion over 45 years -- and a decline in popularity of reunification.
"Younger generations that did not personally experience the Korean War and the initial division of the peninsula do not see the imperative for unification, particularly during an economic downturn," said Kuyoun Chung, a research fellow at the Korea Institute for National Unification.
Opinion polls conducted ahead of South Korea's recent presidential election found that most voters were worried about jobs and the economy more than the country's relationship with the north.
China also has geopolitical and economic interests at play.
A conventional war would likely lead to Kim's ouster if Beijing -- Pyongyang's only real ally -- did not intervene on North Korea's behalf.
It's believed North Korea's policy makers recognize this. Though they will engage in provocative actions like missile launches, they tread with some caution to avoid a military response from adversaries.
"At the end of the day, North Korea's top priority is regime security and regime survival. So it will take every measure possible to make sure that it does not cross any American red lines," said Tong Zhao, a fellow at the Carnegie-Tsinghua Center for Global Policy in Beijing.
Even if the US and South Korea were able to swiftly topple the Kim regime, there would be a series of complications in the aftermath of a war.
Pyongyang's nuclear material would likely be less secure and could fall into the wrong hands. Conflict could spark a refugee crisis. And a change in the region's balance of power could be followed by countries jockeying for more power, fueling instability.
"The result of a North Korean regime collapse would be catastrophic and may trigger a dangerous race between China and the US-ROK (Republic of Korea) forces attempting to secure strategic and symbolic locations such as the Yongbyon nuclear facility and Pyongyang," Andrew Injoo Park and Kongdan Oh wrote for the National Bureau of Asian Research.
China worries about both of those, especially the latter.
Beijing values Pyongyang as a strategic buffer between itself and US-allied South Korea. If North Korea were to fall, it could lead to a US-allied unified Korea, with US troops right on China's border.