Donut-shaped olive-green blasts of smoke grenades create a screen to obscure the approach of tracked armored vehicles that come ashore and disembark fully armed South Korean and American Marines onto a beachhead.

The soldiers instantly storm the quiet, sandy beach in the small east coastal town of Pohang, South Korea.

When the smoke clears, the first wave of troops are in their positions on the beachhead awaiting orders to advance further inland. 

Following the first wave, another set of troops arrive overhead in transport aircraft and parachute onto landing zones to further strengthen the beachhead. The olive drab of military uniforms and vehicles creates a show of force

Dokseok-ri Beach was declared secure by the combined forces and the landing mission was complete in this simulated, joint arms forces operation, dubbed Ssangyong. South Korea and the U.S. conduct the exercise every two years in order to test their capability to execute a mock amphibious landing operation against North Korean aggression.

The two countries kicked off the Ssangyong amphibious exercise for a 12-day run on March 7 and invited journalists to get a glimpse of the elaborate, full-spectrum exercise on Saturday.

Around this time every year, inter-Korean military tensions rise to a crescendo as South Korea and the U.S. roll out their annual springtime military exercises in the southern part of the Korean Peninsula: the computer-based simulated war game Key Resolve and the South Korea-wide Foal Eagle field exercise.

The biennial Ssangyong program is part of the field exercise for which the U.S. brings its key military assets and troops to the Northeast Asian ally.

This year's Ssangyong exercise involved some 12,200 U.S. Marine and Navy forces and 5,000 South Korean troops, the largest number of soldiers the allies had gathered for such an event. 

Nearly 200 military personnel joined from Australia and New Zealand as member states of the United Nations Command, which have been stationed here ever since the 1950-53 Korean War between what later became the capitalist South and the communist North ended in an armistice rather than a peace treaty.

What was designed to be an interim cease-fire still remains today, putting the two Koreas technically at war.

The widely publicized Saturday portion of the amphibious operation drill, a decisive phase involving a landing onto a beach controlled by the enemy set off an unusually scathing reaction from North Korea.

The General Staff Department of the North Korean military, which is equal to South Korea's Joint Chief of Staff, condemned the joint exercise as a "rehearsal for military advance into Pyongyang," in a statement issued Saturday morning. It was the military command body's first official statement ever.

In the statement, the North threatened to launch retaliatory, pre-emptive attack operations in response to the Ssangyong exercise.

This panicky reaction is attributable to the characteristically offensive nature of amphibious operations.

These operations are one of the most complex military maneuvers, requiring an intricate task of putting together special forces, air power, naval gunfire and transports, as well as specialized amphibious equipment. 

Amphibious missions aim to get behind the enemy and secure a military stronghold to project ground and air power from behind the main front in what is tactically called a flanking maneuver.

It was this amphibious assault operation that helped allied forces recapture what is now South Korea from the invading North during the Korean War. 

The landing of massive U.S. forces at the then-neglected port city of Incheon, midway along the peninsula's west coastline, secured a decisive victory for retreating Southern forces to turn the table and recover the capital Seoul. 

In the event of another war with North Korea, an amphibious attack could take place in the initial stages of operations, a military official said, asking not to be named.

"It is intended to hit the enemy in the waist," another Marine Corps official said. "Taedong River, Wonsan and Chongjin could be subject ... any beaches or ports could be possible," he added. 

For this year's Ssangyong exercise, the U.S. brought in two amphibious assault vessels -- the USS Bonhomme Richard and the USS Boxer -- laden with a hoard of seaborne combat jets and transport helicopters, along with some 30 other warships.

South Korea mobilized its own amphibious assault vessel, the Dokdo, in addition to dozens of Assault Amphibious Vehicles, which are carried from vessels to the beach.

This amphibious exercise aims to send a strong political message, as well as a show of force, to North Korea that the South Korea-U.S. alliance is fully prepared to employ the whole range of military power to protect the southern half of the peninsula.

The maneuver also came as military tensions are spiking after North Korea's fourth nuclear test in January, followed up by a long-range missile test the next month.

"At the end of the day, we sincerely believe peace through strength, and it is in the strength of our alliance that we believe can deter and avoid war," said Brig. Gen. John Jansen, the commander of the U.S. 3rd Marine Expeditionary Brigade, which is taking part in the exercise. 

"South Korea and U.S. Marine forces will be the first to arrive at an operation site, and they will execute an operation to attack the enemy's center and secure a pivotal route through aggressive combined power," said Brig. Gen. Lee Seong-do, who commanded the South Korean side of the operation. (Yonhap)