North Korea's acquisition of nuclear weapons and its hell-for-leather development of an array of missiles to deliver them, have raised the stakes further. They called on all parties to de-escalate.
The exercise began hours after Japan's defense minister said the Japanese military could shoot down North Korean intercontinental ballistic missiles before they reach the USA territory of Guam.
President Donald Trump has issued tough warnings to North Korea in response.
A North Korean attack or an American pre-emptive strike is unlikely, said John Delury, an associate professor of East Asian Studies at Yonsei University in Seoul.
As tension escalated this week between the United States and North Korea, the world turned its attention to the USA territory of Guam, which North Korean state media reported had become the target of a retaliatory attack.
"Well, I'm gonna go out on a limb here and say if North Korea was planning some kind of pre-emptive or surprise attack on Guam, we would not be reading about it in North Korean media", Delury said in an interview at his office.
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"In nuclear deterrence, credibility is everything and there's a situation that if no-one takes you seriously, you have to do something to make sure you are taken seriously, and that's where the miscalculation can happen", Yun said. "And there are cases where they (have) made a specific threat and carried it out".
North Korea on Thursday said that South Korea staged a "sordid and mean operation" to avoid the repatriation of the women who used to work at a North Korea-run restaurant in China.
"North Korea better get their act together, or they are going to be in trouble like few nations have ever been in trouble", Trump said, flanked by Vice-President Mike Pence.
Chinese government-backed scholars said Beijing is deeply concerned about the latest statements from Trump and North Korea. "It's a propaganda, that's how I feel", he said but added that the military offered an extra reassurance.
There are, however, political complications in launching attacks from USA bases hosted by allies closer to Pyongyang, should the U.S. retaliate to Kim's latest threats.
Trump echoed that threat Thursday, insisting if North Korea took any steps to attack Guam, its leaders would have reason to be nervous.
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"The rhetoric and actions from North Korea during this period of tension don't mark a significant departure from past periods of tension in my view".
The U.S., China and Russian Federation need to come together to force the North to de-escalate, he said.
US nuclear expert Siegfried Hecker, who has repeatedly visited North Korea's nuclear facilities, said he doesn't think North Korea now has weapons systems for "enveloping fire" around Guam, as it threatened.
"If the United States did try to intercept the missiles they would want to intercept all of them, because failing to intercept them all would send a message about the (US's) limited capacity. those systems aren't flawless", he said.
With hundreds of thousands of troops and huge arsenals arrayed on both sides of a tense demilitarized zone, the Korean peninsula has always been a tinder box. "It's time to tone down the rhetoric".
Bennett said North Korea would be hoping for the United States to fail, but even if the missile defenses failed it wouldn't be a disaster.
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The United States and its allies have two primary missile defense options to intercept a missile between North Korea and Guam. "Making the warhead sufficiently small, light and robust to survive an ICBM delivery is extremely challenging and still beyond North Korea's reach".