The Goryeo-Mongol War Begins
April 1231. Five years had passed.
Goryeo had undergone constant transformations in that time, reshaping the world around it whenever opportunity arose. Foreign merchants who witnessed these changes let out gasps of awe.
"Goryeo's innovations are always so fresh and thrilling."
That was the general reaction. Kaegyeong even had districts where foreigners lived, and travelers roamed the land to admire Goryeo's golden mountains and rivers. The roads had been repaired to a decent standard, making such journeys feasible.
In those five years, road maintenance had been rushed in the south, with auxiliary networks standardized in a fishbone pattern—main roads with dense side branches sprouting off like spines. The north progressed slowly, limited to essential areas. Paved roads would only benefit the Mongols, whose mobility was unmatched; their supply lines would thrive, stripping Goryeo of its mountainous terrain advantages. It would play right into their hands.
Even so, Goryeo had been nurtured meticulously—but D-Day was fast approaching.
In 1230, Choi Hyang, son of Choi Chungheon—my father Choi U's rival in the power struggle—had been caught plotting a coup to seize the regime. He refused to give up. This was only possible thanks to the 'mountain spirit's' intervention.
Mongol envoys visited frequently, spying on Goryeo, making demands, and issuing military threats. Lately, though, they'd gone silent. It felt like the calm before a storm—an ominous sign that could only mean one thing: war.
Goryeo hurried its preparations.
"Make those four wagon fort formations move as one!"
"Jurchen Byeolcho, press the wagon forts!"
"Iron cannon units, commence firing!"
Bang! Rat-tat-tat! Boom!
They maneuvered the wagon forts with fluid precision, firing iron cannons in rapid succession. Of course, it was all for show—no real shots at the Jurchen Byeolcho, lest they friendly-fire their own allies.
The K-Tercio iron cannon wagon fort formation was taking shape.
The soldiers manning the wagons in each formation were elite picks from the second and sixth armies, mostly spearmen from the provinces. Better to train them in advance than scramble ragtag recruits when war broke out.
Each formation, exceeding 10,000 strong, was commanded by a supreme general. Notably, though I wasn't one myself, as the creator of the artillery overseer and iron cannon Byeolcho units, I'd slot into the command chain once fighting started.
Timing being what it was, I quietly tipped off Jusuk.
"You mean remove Jo Sukchang?"
"We can't favor him just because of his father's merits. Right? Recommend Kim Gyeongson for Hamsinjin commander."
"Understood."
I couldn't handle everything personally. Jusuk acted on my behalf without pushback.
At the military council, Jusuk addressed my father.
"We must replace Hamsinjin's commander."
"Jusuk, what do you mean?"
"Unlike his father Jo Chung, who shone in the war against the Khitans, Jo Sukchang is timid, lacks ambition as a general, and would surely surrender if the Mongols came. Summon him back and send another."
Father nodded, clearly sharing the doubts. Should've just said it myself. But handling it all solo would look bad—better let others contribute.
"Who do you suggest?"
"Kim Gyeongson, brother of Kim Yakseon and a righteous general loyal to the state. Entrust it to him."
Father promptly installed Kim Gyeongson as Hamsinjin commander and recalled Jo Sukchang.
In history, such changes needed careful groundwork. Kim Gyeongson had held Gwiju admirably before; he'd manage here too. Unlike the original timeline, Hamsinjin wouldn't hold long—they'd need to fortify Cheolju and clash in the Gwiju Plains.
Assuming a Mongol invasion, the military leaders rushed defenses.
This was unexpected. Even the once-incompetent martial regime assessed the situation coldly, diverging sharply from history.
"Assemble the northwestern provincial Byeolcho to bolster Hamsinjin to at least 5,000. Entrust this to General Park Seo of the Northwestern Military Command. And reinforce the fortress, as it's vulnerable to large forces."
Station 5,000+ at Hamsinjin to repel invaders. Kim Gyeongson had blocked Mongols with 5,000 at Gwiju historically—feasible here too, first battle aside. Ample grain and weapons stocked the northwestern forts.
"Cheolju has 2,500 soldiers, but lacks grain and arms. We must supply them."
Even Cheolju, which historically crumbled from exhaustion, now received plenty.
"For battle in the Gwiju Plains, stock Gwiju fortress with grain, weapons, and troops for supplies."
"To make northwestern forts endure, install artillery on the walls, no?"
Artillery on walls. Goryeo had amassed gunpowder weapons, from self-produced niter beds to imported sulfur. The northwestern Byeolcho had artillery training.
The proposal passed easily; batteries rose on Hamsinjin's ramparts.
"For field interception, Goryeo's maximum rotating force is just over 100,000."
"Prepare supplies for 100,000 for at least three months first."
Agricultural surpluses covered it.
"If war erupts, Supreme Generals Daejipseong and Jusuk will command the Night Byeolcho to defend Kaegyeong."
"Can't this general join the field?"
"If they use the direct march strategy, Kaegyeong's at risk—the royalists might stir. My closest vassals must guard it. You haven't forgotten the emperor's collusion with the Mongols?"
Conveniently, history's bungler Daejipseong stayed for Kaegyeong defense. If the central army shattered, the capital would fall anyway—but by then, Gojong would bolt.
Kaegyeong couldn't be neglected. If the emperor issued a late edict during war, begging the suzerain Mongols to crush the traitorous Choi regime, the invaders would become imperial liberators. No pretext against Mongol forces. They knew Goryeo's state from constant envoys. Kaegyeong needed urgent fortification.
"We obey."
"Park Songbi, monitor the palace rigorously."
"Yes, my lord."
Clear enough. But one issue remained.
"Father, what will you do? Shouldn't you oversee Kaegyeong's defense?"
"Telling Yi Ja-seong and Chae Song-nyeon your strategies now won't suffice to lead the whole army."
True enough. Yi Ja-seong, Chae Song-nyeon, and Kim Hui-je knew the tactics but had only recently drilled them.
"Father, personally?"
"Yes. I'll lead myself."
Father hated yielding glory. This feels like it's gonna go tits up. Is it okay?} No real flaws in his logic. The others hadn't heard strategies straight from me like he had. But was it right? Risky as hell.
Time passed.
"Report! My lord! Mongol forces! They've massed beyond the Amnok River!"
The news of Mongol camps near the Amnok reached the council. War had arrived. The world's strongest army bared its fangs at Goryeo.
Father and I hastened to the palace for audience.
"Your Majesty, scouts report Mongols camped beyond the Amnok."
"I-is that so? Shall we seek peace? Or fight? Words first, surely."
Gojong eyed avoidance, but we'd built armies for this. Sucking up would just delay the inevitable.
"We've prepared for this day. We'll raise 100,000 and crush them in the Gwiju Plains before they cross the west capital."
Father's bold declaration.
"How will we deploy? Who's commanding?"
"I'll lead with Northwestern Military Commander Chae Song-nyeon and Supreme General Yi Ja-seong."
"The Prime Minister personally?"
Gojong's eyes darted schemingly. This bastard's plotting nonsense again. To us, he was the villain. Royals saw Choe as usurpers ripe for devouring—but as the targets, we wanted him gone. Just sit pretty like history. What wind blew this trollery?
"Yes. No worries. 5,000 Night Byeolcho will defend Kaegyeong against contingencies. With your edict, please."
Choe troops guarding Kaegyeong—don't even dream of nonsense. An edict commits him; no backing out to the Mongols.
"But they could seize Cheolju and march south."
"Why retreat from a field battle favoring us? They'll aim for the main army. Even Mongols won't split against 100,000."
Some argued internal defense, but scouts reported 20-30,000 enemies—far fewer than the Khitan war. Consensus: one decisive field battle feasible.
Not long before, the Mongols' fateful hour struck.
"Salitai, as Rights Emperor, take 30,000 troops and invade Goryeo! Drag out that Choi U and force surrender!"
Khan Ogodei appointed Salitai—trusted by Chinggis from the Iribu—to lead the 1231 lunar August invasion with 30,000.
"Grand Commander, can tiny Goryeo defy us?"
"Who under heaven withstands Mongol arms? They serve the dying Jin as suzerain. They'll kneel soon enough."
Salitai dismissed Goryeo—a minor state clinging to rugged borders. Goryeans might protest their past deference, but Mongols sought total subjugation, lord-vassal bonds.
Thus, the Mongols reached Hamsinjin.
"Who's Hamsinjin's commander?"
"Defender-General Kim Gyeongson."
"A novice without war experience. Offer surrender."
"Yes, Grand Commander."
Unlike history, Kim Gyeongson commanded reinforced Hamsinjin with 5,000.
He scoffed at Salitai's demand.
"30,000 they claim, yet this Salitai treats us like fools. Barbarians fresh across the border demand surrender? Laughable."
"Indeed, General. Utterly arrogant."
"We've eaten the state's rice, watching winds. Now chance to die for country—how could a general hesitate against barbarians? I, Kim Gyeongson of Hamsinjin, will lead soldiers and people against them!"
Rejection delivered.
"They spurn Mongol mercy. Goryeo placed a decent man upfront. Fools. Prepare battle! I'll face this Kim Gyeongson myself!"
The opening battle of Hamsinjin—the prelude to Mongol-Goryeo war—began.