It took only a short while.
Xue Rengui learned about Pei Xingjian's family history—his father's and elder brother's past deeds, his ancestral home, personal interests, residence in Chang'an, and ambitions in life.
Pei Xingjian learned that Xue Rengui's original name was Xue Li.
Turning a deaf ear to the incessant chatter at his side, Xue Rengui studied the sand table with unusual focus.
His Majesty had spoken clearly. Though he did not know for certain whether this involved Tuyuhun, Xue Rengui still wanted to prepare in advance. Thus, he forced himself not to listen to Pei Xingjian's ceaseless babbling and instead widened his eyes, carefully observing the shapes of mountains and rivers laid out before him.
Yet as a standard example of a fallen household, even with his best effort, Xue Rengui could only imagine two groups of tiny figures clashing and killing one another across the terrain.
Beyond that, he truly had no further insight.
And just at that moment—
"Have you seen anything yet?"
A kindly voice sounded beside Xue Rengui. Absorbed as he was, he answered without thinking,
"The shape of this hill looks rather like that steamed bun."
The moment the words left his mouth, Xue Rengui realized something was wrong. He looked up, only to find that at some unknown point, two more people had entered the room—one an elderly man, the other, judging by his features, about thirty.
The one who had spoken was the elder. His robes bespoke rank, his face warm and gentle.
Xue Rengui's answer caused the old gentleman to break into a broad smile. He turned to the man beside him and said,
"Dingfang, it seems this divine general from Hedong has an appetite well suited to Guanzhong cuisine."
Su Dingfang smiled faintly. After carefully sizing up the visibly uneasy Xue Rengui, he introduced the two people present:
"This is Duke Li Jing of Dai, Doctor of Military Studies."
No battlefield achievements were needed to adorn that name alone—it already thundered in the ears.
Xue Rengui dared not be slow to react and hurriedly saluted. Beside him, Pei Xingjian followed suit.
Li Jing showed no ceremony. He picked up a long, thin bamboo pointer and tapped a large lake marked on the map.
"This is Qinghai Lake. When the grand army sets out next year, this place must be fully controlled—to sever the enemy's strength and to enable distant designs upon the Western Regions."
"Around this lake, Tuyuhun occupy the west, south, and north; to the east are the Dangxiang tribes. Further west, upon the plateau, Tubo is also stirring restlessly."
"Xingjian—what do you think should be done?"
The gazes of all three present converged on the short Pei Xingjian. After all, he was only eleven years old.
Xue Rengui was equally curious. The sense of unreality he felt now surpassed even that of yesterday.
Names like Tuyuhun and Dangxiang were things he had only heard from village elders telling stories.
As for Tubo or Qinghai Lake—he had never even heard of them.
Yet listening now to Duke Li's grave words, combined with His Majesty's hints from the day before, Xue Rengui understood all too clearly:
Next year, he would be heading to this place called Qinghai Lake, fighting life-and-death battles with the Tuyuhun and Dangxiang peoples.
He felt only one thing—
It was happening too fast.
As though for the first sixteen years of his life he had walked on his own two legs, and now the Emperor of Great Tang had forcibly thrust him onto a fine steed, compelling him to gallop forward at full speed.
Pei Xingjian, however, showed no fear. He carefully examined the sand table and finally said,
"This student has also heard that the Duke once led light cavalry in a surprise strike at Yinshan, settling the battle in a single engagement. Our Tang armies then encircled from three sides, destroying the tribes of Jieli."
"But Tuyuhun is not the Turks. Our Tang armies come from the east and can engage on only one front. If this sand table is accurate, to imitate the strategy used against the Turks would require passing through Hexi, circling into the Western Regions, then turning south to form a pincer against Tubo—this is simply impossible."
"Therefore, to control Qinghai, we should befriend the near and strike the far: use wealth to bribe the Dangxiang, then have the main army launch a surprise assault directly against Tuyuhun. Each soldier should be issued three horses, pursuing and annihilating this enemy."
"Once Tuyuhun is pacified, the Dangxiang will be no more than a threat under Tang's armpit."
Watching the eleven-year-old Pei Xingjian speak with such fluency, Li Jing's face was filled with delight.
Worthy indeed of the old Pei family—father and elder brothers all fierce generals. Though they died early, constant exposure at home had given him keen insight into military affairs.
Moreover, perhaps due to his background in the Xiuwen Academy, his associations were broad, and he possessed a solid grasp of current affairs and geography. To already hold such views—
At this moment, Li Jing glanced at Su Dingfang and fully understood why, in history, Su Dingfang would later teach Pei Xingjian the art of war.
What an outstanding seedling.
Your disciple is excellent—but very soon, he will become a successor to the Wei Gong Art of War.
Su Dingfang's face was filled with pain.
The more outstanding Pei Xingjian proved, the more his heart ached.
Seeing that the boy had already reached such understanding at merely eleven years old, Su Dingfang felt as though his heart were bleeding.
What a fine seedling.
But—
Su Dingfang also cast a glance at Li Jing.
Why does General Yaoshi get to compete with His Majesty for talent?
Because of overwhelming merit.
As for warfare—he, Su Dingfang, could fight as well.
And military merit? Were Tang's surrounding lands not all waiting to be pacified?
Thus, at this moment, a surge of heroic resolve burst forth in Su Dingfang's heart.
To hell with leaving a name in history—that was secondary.
He, Su Dingfang, would definitely snatch his disciple back!
Xue Rengui felt his heart sink a little. He looked at Pei Xingjian, who barely reached his chest, then at the sand table.
Why couldn't I see the characters written on this sand table?
Fortunately, the topic soon passed. Li Jing clapped his hands lightly and explained the situation to them.
The military studies at the Guozijian were currently only in a trial phase. Li Jing served as Doctor of Military Studies, with Su Dingfang as Assistant Instructor.
At present, there were only two students: Xue Rengui and Pei Xingjian.
However, His Majesty had already issued edicts throughout the realm to tentatively establish a martial examination.
Those who distinguished themselves in the martial examinations would likewise enter this military course to study the art of war.
Pei Xingjian would still continue his courses at the Xiuwen Academy, only coming here every few days to attend lectures.
Xue Rengui, however, received special attention.
After this lesson ended, and after confirming that Xue Rengui could read, Li Jing handed him a bag of books.
"Bestowed by His Majesty. All must be read."
Xue Rengui craned his neck to look. His vision swam a little—inside were packed volumes of military texts, and… mathematics?
Seeing the question in Xue Rengui's eyes, Li Jing nodded firmly.
"His Majesty intends to promote mathematics. Before long, it will be listed as a required subject in both the Xiuwen Academy and the Guozijian."
In that case, no problem.
Xue Rengui set aside his doubts and accepted the books with reverence.
He had already asked—food and lodging at the Guozijian were fully provided. There was no need for him to seek out a livelihood.
If so, then he would study properly.
If for no other reason, that Pei Xingjian was simply far too dazzling.
Watching the divinely appointed general carefully cradle his military texts as he went off to find lodgings, Li Jing was full of satisfaction.
"Both are materials that can be shaped."
Su Dingfang nodded in agreement. Who could deny it?
Afterward, the two of them turned and headed for the Wulei Monastery.
Later generations often joked that Kuangdao Prefecture was a place for sweeping floors—showing how well known it was that the Kuangdao Prefecture truly had nothing to do.
In such circumstances, Su Dingfang chose to follow Li Jing, intending to go see that gunpowder as well.
Yet after leaving the room, the first person they encountered was the high-spirited Yan Lide.
Li Jing greeted him,
"Judging by your expression, Master Yan, it seems your lecture went smoothly."
Yan Lide clasped his hands gratefully. After the experimental ignition of gunpowder north of the city, his first thought had been to start with the Directorate of Works to mass-produce it.
He had expected Li Jing to object, but instead the Duke of Dai proved broad-minded, accompanying him to see His Majesty to discuss the matter of gunpowder.
Now, the Wulei Office under the Directorate of Works had already taken initial shape—all thanks to Li Jing's assistance.
Thus, Yan Lide responded warmly,
"The Duke looks radiant—so Xue Rengui and Pei Xingjian have both lived up to their reputations?"
After a moment's thought, Yan Lide produced an item.
"This is the yangsui I used in teaching. The Duke should find it useful."
Li Jing accepted it curiously. It felt cool to the touch. This small piece of glass had been polished so that every surface bulged at the center and dipped at the edges.
Holding it in his hand, he discovered with surprise that through this oddly shaped glass, the lines on his palm appeared much larger, astonishingly clear.
Still, Li Jing asked curiously,
"This is also called a yangsui?"
Li Jing was not unfamiliar with yangsui, as Daoists often used them in alchemy.
They were not complicated to make—cast copper into a concave shape, polish the interior smooth, face it toward the sun, and light would gather within the hollow. Hold dry grass near it, and it would ignite.
Occasionally, armies still carried them on campaign, in case they fell into dire straits.
But after sealing the Wulei Monastery, Li Jing and Li Shiji, while strictly controlling sources of fire, had simply banned even yangsui altogether.
This glass yangsui, however, differed greatly from the copper ones.
Yan Lide demonstrated its use on the spot—placing it under sunlight to focus the light to a point, adding dry grass, which soon ignited.
"This thing is indeed convenient," Li Jing said.
At a glance, it was obvious—this was far more convenient than copper yangsui.
Not only was it easy to carry, the magnifying effect alone delighted Li Jing.
Moreover, as he looked up at the sky, a thought suddenly occurred to him—
Could this not also be used for timed ignition of gunpowder?
For instance, if intelligence reported that bandits would pass through a certain place at the second quarter of the wei hour tomorrow, one could bury gunpowder in advance at night and set the yangsui according to the sun's angle at that time…
Though such scenarios might be rare, there were surely many other applications.
And Li Jing realized that this, once again, involved mathematics—truly the king of sciences.
As he rubbed the glass yangsui in his hand, Li Jing turned to ask about something else he cared about even more.
"And the telescope?"
Yan Lide sighed.
"It still requires waiting."
The "method that would make one's name last a thousand years" he had proclaimed when lecturing at the Guozijian was precisely the telescope.
To put it nicely, it was because "many minds make light work."
To put it plainly, the Directorate of Works was running out of manpower.
The Wulei Office had just been established; naval matters required research; weapons and armor from later generations, as well as innovations in horse armor, needed experimental production; steelmaking methods required improvement.
And now they also had to figure out how to mass-produce iron nails.
Since the founding of the dynasty, the Directorate of Works had never been this busy.
Thus, after prioritizing tasks, Yan Lide simply chose to let Guozijian students work for free—for the good of the state.
After all, researching a telescope was not like smelting iron.
Iron smelting required blacksmith experience, furnace-side experimentation, and support from ironworks.
A telescope, in theory, only needed several pieces of well-polished transparent glass to begin experimenting.
Thus, Yan Lide sent over the polished glass, gave vague guidance on direction, and let Guozijian students attempt targeted trials.
At the same time, he also planned to remonstrate with His Majesty—
Perhaps the Directorate of Works, too, needed its own academy.