His comments came as he visited the border between North and South Korea on Oct. 26.
Mattis also reassured South Korea that the United States would stand with its close ally in the face of threats from the North.
“We stand shoulder-to-shoulder with you, with your soldiers, and with your people in confronting the threats posed by the Kim Jong Un regime,” he said.
The North Korean regime has threatened multiple times in recent months to strike the United States, South Korea, and Japan with nuclear weapons.
The CIA believes that Pyongyang is just months away from solving engineering challenges that will allow it to launch a ballistic missile armed with a nuclear warhead.
“Should they ever use it, they will be faced with the strong might of the [Republic of Korea]–U.S. combined forces, and they will be met with a proportional and firm response,” said South Korean Minister of Defense Song Young-Moo while accompanying Mattis at the border.
In his own speech, Mattis drew a distinction between a free South Korea and an oppressive North.
“To the south lies a vibrant country, a vibrant economy, a free country, and it’s underpinned by peace-loving members of a free society,” he said.
“Behind me to the north, an oppressive regime that shackles its people, denying their freedom, their welfare, and their human dignity in pursuit of nuclear weapons [and] the means of delivery, in order to threaten others with catastrophe.”
Following World War II, Soviet troops controlled the north of Korea and U.S. troops, the south. Soviet troops withdrew after the Communist Party of North Korea was installed, with Red Army-trained Kim Il Sung at the top.
Since then, the North has been subjected to horrific communist rule, whose failed economic and social policies have brought the North Korean population great suffering. To date, much of North Korea has no or only limited access to electricity, and millions have died as a result of famine. Those with religious beliefs are subject to extreme forms of torture.
South Korea, meanwhile, has become one of Asia’s most prosperous economies, with its citizens enjoying one of the highest standards of living in the world.
“North Korean provocations continue to threaten regional and world peace,” Mattis said.
Tensions escalated on the Korean Peninsula after North Korea conducted its sixth underground nuclear test in September.
President Donald Trump has repeatedly called on North Korea to abandon its nuclear weapons program. During a speech before the U.N. General Assembly in September, Trump said that while the United States has great patience, it could “totally destroy” North Korea if it is forced to defend itself or its allies.
Mattis has made similar statements, saying that while the United States does not seek to attack North Korea, it has means at its disposal to “completely annihilate” the country if absolutely necessary.
The United States currently has three aircraft carriers and their associated missile-carrying vessels deployed in the Western Pacific Ocean.