The wargames, details of which are a closely guarded secret, simulate military conflict with the isolated country. South Korean President Moon Jae-in said that its neighbor is crossing the red line with these tests.
According to the Pentagon, at least 17,500 U.S. service members would be taking part in the exercise, as well as troops from other countries such as Australia, Colombia, Canada, New Zealand, Denmark, the Netherlands and the United Kingdom.
The US military describes the software behind the drills as "state-of-the-art wargaming computer simulations".
Mattis stressed, however, that the Trump administration is pursuing its goal for a nuclear-free Korean Peninsula via diplomatic means.
"You can expand a certain area to see what troops are in what sort of status and where they will move", said Park.
That focus on the North Korean leadership is what particularly infuriates Pyongyang, experts say.
Japan supports India's stand on Doklam
In mid-June, Chinese soldiers attempted to construct a motorable road in Doklam which was stopped by their Indian counterparts. It is not that China is eager to form a military alliance with India.
"We will follow to the ends of the earth those who dare try to harm our core".
29 July 2017 North Korea launched an Intercontinental ballistic missile Hwasong-14.
US Department of Defence said the Ulchi Freedom Guardian drill is a computer simulated defensive exercise created to enhance readiness and maintain stability on the Korean peninsula.
North Korea said it is holding fire on Guam but is watching the action of the U.S. before deciding to hit the American territory with missiles.
Two days ago, President Donald Trump described North Korea's decision to delay any military action against the USA territory of Guam as a "very wise and well reasoned decision".