The tanks pound the hills around Pocheon, about 25km from the demilitarised zone, the border which separates North and South Korea. Moments later, the attack comes from the air, as fighter jets, then helicopters, swoop in. Flashes of orange are seen in the distance, then grey clouds of smoke rise up, as the bombs hit their targets. The ground reverberates with every explosion.
This is South Korea’s largest ever live-fire drill with the United States, staged to commemorate 62 years since the Korean War started on June 25, 1950.
2,000 troops, tanks, fighter jets, rocket launchers are involved in the exercise, which coincides with two other military drills: one just south of the Korean peninsula with Japan, US and South Korea, and the other with the US and South Korea, off the west coast of the country.
South Korean defence ministry officials say the drill is intended to display the military’s combat readiness.
“With cutting-edge equipment and strong combat power, our military is fully ready to deter the enemy’s provocations and respond immediately in the case of an actual attack,” says Colonel Park Jeong-Tae of the Republic of Korea army.
"We train together to ensure we are ready to stand shoulder-to-shoulder with our [South Korean] army partners if called upon,” Lt Col Joe Scrocca, a spokesman for the US Army's 2nd Infantry Division, tells the Stars and Stripes, the US military newspaper. “We are a more lethal force together than we would ever be separate.”
The message really is for North Korea, with whom the South is still technically at war - the two having agreed to just a truce in 1953 and not a peace treaty.
North Korea routinely denounces such drills as provocations.
"Dark clouds of a new war are thus hanging heavily in Northeast Asia, including the peninsula," warns a piece in the North Korean ruling party newspaper Rodong Sinmun,
But analysts say this and the other military exercises are unlikely to provoke further reaction from North Korea for several reasons. The country is going through one of its worst droughts, and food, especially in rural areas, is scarce. The UN is asking for nearly $200m in donations to feed North Koreans.
The other reason has to do with politics in South Korea.
“With a presidential election coming up at the end of this year, North Korea won’t take physical or military action” says Yang Wook, a researcher with the Korea Defense Forum. “Real action could come after the presidential election.”
A response by North Korea before then would risk drumming up greater anti-North Korean sentiment among South Koreans. And that could decrease the chances of seeing a more progressive politician, someone who’s likely to have a more sympathetic view of North Korea, elected to office.
Under former President Kim Dae-Jung, South Korea extended aid, with no questions asked, to its neighbour. The Sunshine Policy, introduced in 2000, tried to use economic assistance and passive language to reach out to the communist North. It was a policy kept by his successor, former President Roh Moo-hyun.
But when Lee Myung-Bak took office in 2008, the unconditional aid stopped. Lee’s stance has always been to insist that North Korea give up its pursuit of nuclear weapons in return for food and other aid. Two years ago, the South Korean Unification Ministry's annual report described the Sunshine Policy of peaceful engagement with North Korea a failure.
But things may change yet. Earlier this week, a man who is most likely to revive the Sunshine Policy, Moon Jae-in, formally announced his candidacy. He once served as Chief of Staff to Roh.
North Korean leaders won’t just be monitoring the war games closely. They’ll be keeping a close eye on elections in South Korea. They have more at stake then.