Precursor to the War
A previous war was fought in the Korean peninsula in the year 2018, ending with severe consequences for North Korea and a major rearrangement of the North Korean Party. However, as though "inefficient" as the North Korean Party could come, they still think that their aim of seizing complete control of the Korean peninsula was still possible, given that the 'American bandits' would go away for good and they could enact major reforms in their country. They started doing the latter after the previous leader of North Korea, Kim Jong-Un, was deposed.
As North Korea enacted reforms on its economy, which included privatizing much of its industry and distancing itself from Communism even further, some analysts saw the great opportunity to unite the divided Korean peninsula without bloodshed. With the North Korean economy revitalized with Chinese help, South Korea started sending diplomatic teams to Pyongyang in order to invite them for a dialogue about unification. Although realistically an impossible scenario, peace was of the best interest of both parties. For South Korea, it meant that it will free them from costly defense policies and a larger buffer zone against Russia and China, the greatest of the three devils.
However, with the American forces retreating back to CONUS in the year 2028 following the GLA raid on their mainland, North Korea saw a chance that might never come back to them again; full control over the Korean Peninsula, with North Korea at the helm. With the new land and manpower, an invasion of Japan and the Philippines would be feasible, allowing Korea to carve out its own empire in the Pacific and achieve Hideki Tojo's vision.
North Korean Rush
Nobody could still say how or when did the war actually start, but the very first call of attack was by a scared private who said that the South Korean fortifications on the border were "annihilated" and large masses of hostile vehicles were coming to their location and that he requested instructions. In a matter minutes, the entire military hierarchy was alerted, but very too late to save the border guards. The very first sign of resistance by South Korean forces was on the outskirts of Seoul. Several ROK units blocked all roads towards the direction of Seoul, but they were smashed, or rather, burned to the ground by air support planes as the ROKAF was struggling to get into the air due to DPRK commando raids. After a measly three hours after the first alert came, North Korean units were in sight of the South Korean capital. Luckily, chaos did not ensue, and what happened was an orderly evacuation of women, children, and other noncombatants while able-bodied men were given assault rifles, AT4s, grenade launchers, submachine guns, pistols and even baseball bats to defend the capital for as long as possible. Thus, the battle for Seoul begun.
Defense and Capture of Seoul
Due to the inherent lack of heavy weaponry in Seoul armories, the defenders of Seoul, numbering only at at least 2,000 regulars and a lot of hastily-armed civilian reserves, were hard pressed to defend against the shock forces of the DPRK, consisting of light infantry in BMP Transports and old T-64 Tanks, in conjunction with Type 88Gs. Luckily for them, however, several ROKAF support units were at Seongnam Air Base for a training mission, and they managed to defend the skies around the defenders against MiGs, in addition to providing limited air support. This combination managed to hold on Seoul for three days, until the heavy elements of the North Korean army came, bringing with them advanced Chinese weaponry, including ECM Tanks, Twin Fangs, and Inferno Cannons as well lifting the previous restrictions to take the city intact. Few hours after the Inferno cannons were put in place, they and the hundreds of thousands of North Korean artillery pieces opened fire, over the course of an hour Seoul would suffer heavy damage through a non-stop bombardment that would end with much of Seoul being reduced to rubble. After the bombardment, lead elements of the NK Army advanced onto the city and after fierce fighting with the remaining defenders seize it, despite heavy damage to the city itself, North Korean forces would manage to capture a number of prisoners including the South Korean President, who vowed to stay until the last man in Seoul were evacuated, and his cabinet ministers, save for the Minister of National Defense and his personal team, since they were in Pusan before the war began. The president was publicly executed by firing squad hours later, having been "accused" of "crimes against Kim Il-sung". The action was condemned by not only China, but also by Japan, America, Australia, and even Russia, calling it "unnecessary to accuse someone of committing crimes against a man who died in 1994". Some in the American public called for American military intervention, but the government refused, fearing that the GLA would exploit there suddenly being fewer soldiers on American soil and launching a major attack on a major population center.
Drive to Taejon
After Seoul, the North Koreans eyed another strategic location, Taejon. The second-most important nerve center of South Korea after Seoul, it housed the last remaining command structures of the entire South Korean army. Its capture, some advisers reasoned, would lead to the capitulation of the entire South Korean army as a whole. However, the road towards Taejon was very arduous for the NK army, as several ambushes led by South Korean special forces tied up traffic on the major roads. By this time, it was already spring, and the tanks, built for open ground, were having a difficult time moving across the mud-bogged fields. For the South Koreans, this meant more preparation time against the numerically superior enemy.
The North Korean army was significantly slowed at the banks of the Imjin river by South Korean regulars, and most of the second-echelon units were tied up behind the Han river because of South Korean special forces, and stay behind units, thereby earning this part of the campaign as "The Battle of the Two Rivers".
Air War on the Road to Taejon
South Korea fully utilized its superior air force against the North Koreans. Aside from close-air support and mobile artillery, they used their old yet respectable AC-130s "Spectre I" Gunships in SEAD role as well, absorbing immense flak fire while their more advanced jets cleanly sliced through Twin Fangs, Gattling Tanks, and other anti-air units.
However, the biggest hurdle that the ROKAF faced was North Korea's massive number of fixed-wing and rotary aircraft, mainly Helix helicopters, vintage MiG-21s and hundreds of obseclecent 3rd and 4th Generation fighters that came from China, although there were a large number of dubiously-acquired Gazelle choppers. Although ROKAF's F-22B Raptors, F35s, and other craft were clearly superior in quality against North Korea's machines, the fact that the North Korean air force outnumbered the ROKAF by least 3:1 in the opening stages of the war offset the technological advantage.
Another thorn in the back of the ROKAF was the presence of Chinese AWACS planes, which provided target information for the North Koreans and caused major problems for any ROKAF pilot. Moreover, the South Korean Minister of Defense specifically barred any pilot or SAM battery commander from destroying these planes in fear of Chinese escalation, which would spell doom for the nation and much of its civilian population, given China's track record during the Global War on Terror.
Over this part of the campaign alone, the South Koreans had over 2 dozen aces with 6 double aces and 1 triple ace, due in part of the relatively inefficient use of air assets by North Korea. With kill ratios being almost 4 to 1 in favor of the ROKAF due to their more advanced training and hardware along with the relatively inexperienced North Korean commanders due to loss of their experienced veterans in the 2nd Korean war, which would allow for ROKAF pilots to bait their opponents into well-placed ambushes or SAM traps. The ROKAF and later JAF would prove to be vital during serving with distinction over the course of the war keeping the skies clear and providing vital support for ROK and later Japanese troops in hundreds of engagements during the conflict.
Siege and Capture of Taejon
When the lead North Korean elements reached Taejon, its defenses were considerably complex and nearly impenetrable; layers and layers of buildings specifically designed as garrisons littered the city, not to count the apparent willingness of the ROK army to expend their supplies profligately. Even with significant fire support, the defense circling Taejon proved unbreakable.
Knowing that another stalemate would mean another strategic defeat, China allowed the transfer of their 'weapons of mass destruction', specifically Meteor Nuclear Cannons, and blueprints for EMP Bombs. Meanwhile, North Korea unleashed upon Taejon its viable stocks of biological and chemical weaponry, mostly consisting of dubiously-acquired anthrax, mustard gas, VX, and lewisite all loaded in old theater ballistic missiles. Because of this escalation, The defense of Taejon would be broken as any attempt to hold the North Korean army would be broken by Tactical nuclear weapons. Taejon would fall after 3 days of defense. With the loss of much of the garrison and well over 90% of its pre-war population due to disease, chemical weapons, and radiation. Not including the near-total destruction of the city due to the liberal use of weapons of mass destruction and making the city inhabitable for decades due to the presence of Anthrax Spores.
South Korea Asks for Help
Faced with complete and total annihilation against weapons of mass destruction, and unable to attack Chinese forces out of fear of Chinese intervention and possible deployment of strategic nuclear weapons. South Korea asked for help from anybody in the entire Asia-Pacific Region, counting, India, Japan, the Philippines and even Australia. But with Australia facing domestic problems and crackdowns, India facing a problem of its own in its borders with China, and the Philippines facing a potential Taiwan-esque naval confrontation in the long-contested South China Sea, only Japan was in a position to help South Korea in any meaningful way. However, China made many attempts as to stop Japanese intervention in the war, including offers of a special long-term trade agreement, the secession of the Diaoyu/Senkaku islands, reminding them of what transpired in World War II as a scare tactic, and even bilateral strategic cooperation.
Japan's "Covert Intervention" turns to Complete Participation
In reality, Japan was already helping South Korea in a rather indirect way; they covertly provided supplies to the ROK army through the Tsushima Strait, using neutrally-flagged transports, and maybe even submarines, if rumors that circulated during the war was true. Diplomatically, Japan slammed North Korea's "War of Aggression" and China's "virtually belligerent participation" in the war. However, at first, they really weren't serious about entering the war directly, in fear of severe repercussions from China, destabilizing the fragile status quo between Japan and China. Though the issue of violating the Constitution was no longer an issue thanks to reforms that allowed Japanese forces to go to war to defend an allied country. Along with reforms after the withdrawal of US forces that transformed Japans self-defence forces into a military twice, it's size.
With North Korea's use of weapons of mass destruction, Japan feared that if their "covert intervention" was exposed, they might be attacked by North Korea suddenly and without any direct provocation. They especially feared being attacked by nuclear weapons, having already bore witness to them at the end of WWII. Because of this, they entered the war as to preempt the North Koreans from using their weapons of mass destruction against Japanese civilians. The entry of Japanese forces was known as Operation Reclamation. During this hour of desperation, the South Koreans found help from an unexpected ally: The island nation of Japan. Together, the two former enemies, now united against a common adversary, reclaimed the initiative and prompted the Chinese to withdraw their support due to a variety of socioeconomic reasons along with tensions along the Sino-Russian border.
In 2032 the joint militaries of Japan and the ROK began the long-awaited counteroffensive. It began with a major air campaign by the ROKAF and the JAF against key targets such as command centers, supply depots, airfields, roads, and bridges.Then followed by major artillery bombardment against North Korean positions. Minutes after the last shells had landed allied forces began their offensives smashing through the North Korean lines as ROK and JF forces focused their effort on only a handful of points on the North Korean line. Six hours after the first ground units began to move out, North Korean lines had broken at a dozen different points from Gunsang to Pohang. When the military command of the North Koreans finally heard about the counter-offensive they attempted to counter attack only to find that their reserves either had already been ravaged by JAF and ROKAF forces or were already engaging as ROK and Japanese armored units ran riot in the Norths rear lines. By the time the order to retreat came it was too late for many units of the North Korean Army with thousands surrendering in mass. Most were arrested as war criminals due to their actions. several days after the operation had commenced it was clear to the world that the tide had finally turned for South Koreans.
The Road to Pyongyang
In late 2032 the joint ROK and Japanese forces have pushed the North Koreans out of South Korea and began on the road to the Yalu River the ancient border of the Korean peninsula. With the loss of much of the North Koreans professional army in the South, the only resistance came from North Korean reserves and the remaining units of the North Korean armed forces. It did not take long for the Coalition forces to be advancing on the gates of Pyongyang
On January 3rd, 2033, the coalition forces liberated the Northern capital city of Pyongyang. But their victory was a Pyrrhic one; the entire country had suffered heavy damage to its infrastructure, along with countless military and civilian casualties. The only true victor was China, as it had managed to both eliminate a long-term political liability in the North and cripple a confident economic competitor in the South through its ruthless manipulation. With Regardless, the people of Korea now found themselves in a shared misery and went on to rebuild their newly reunited nation, which now became the United Republic of Korea, together. For the first time since 1910, 123 years prior, Korea was once again a single, unified, sovereign nation.