The United States and South Korea's joint military exercise kicked off on Monday, despite opposition from North Korea. The drill is meant to boost South Korea's defense for possible North Korea armed threats.

The military exercise called Ulchi Freedom Guardian (UFG) is a yearly military drill between the two countries. This year, 50,000 South Koreans and 30,000 U.S. soldiers participated in the exercise and adopted the "tailored deterrence strategy" which was introduced last October.

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The strategy aims to prepare South Korean forces from North Korea's possible threats of nuclear weapon. The computer-assisted simulation will run for 12 days.

North Korea has been opposing the annual joint military exercise.

Last year, the UFG clashed with the scheduled reunion of family members separated by the Korean War.

North Korea reportedly insisted that South Korea should delay it until after the reunion, but the exercise resumed as planned. Surprisingly, North Korea allowed the reunion to push through.

This year's exercise, however, received a graver reaction from Pyongyang. They alleged that the joint exercise is a preparation for invasion.

As a preemptive response, they will launch attacks as simulation occurs at any given time, according to the General Staff of the Korean People's Army.

In 2013, Pyongyang made the same threats against the UFG, increasing the military tension between the two countries.

North Korean Committee for the Peaceful Reunification of Korea said in a statement on Thursday that the joint exercise must be "cancelled unconditionally" to show Seoul's sincerity in strengthening ties with them.

On the same day, North Korea fired short-range missiles during Pope Francis' visit to Seoul.

The U.S. has been an ally of South Korea since the Korean War, while China allied with North Korea. Five million soldiers and civilians died during the war. The two countries remain divided until today.

TagsUS-South Korea military drill, ulchi freedom guardian