The exercise was tagged “show of force”, a military operation intended to warn or to intimidate an opponent by showcasing a capability or will to act if one is provoked.

The Air Force B-1 bombers, which were escorted by South Korean fighter jets, dropped the bombs in response to North Korea’s missile launch earlier this week.

The Korean Demilitarized Zone is a strip of land running across the Korean Peninsula.

It was established by the provisions of the Korean Armistice Agreement to serve as a buffer zone between the Democratic People’s Republic of Korea (North Korea) and the Republic of Korea (South Korea).

The zone is a border barrier that divides the Korean Peninsula roughly in half, which was created by agreement between North Korea, China and the UN in 1953.

Mattis quoted Gen. Vincent Brooks, the Commander of U.S. Forces in Korea, as saying America and South Korea have exercised extreme self-restraint in avoiding war.

He noted the shelling of South Korea’s Yeonpyeong Island in 2010, the sinking of a South Korean ship earlier that year and other provocations at sea, on land and in cyberspace.

“Our self-restraint holds, and diplomatic efforts remain underway as we speak”, he said, adding that diplomatic and economic efforts remained the tools of choice to convince North Korea to stop its nuclear and missile programmes.