Xue Li listed his letter as an urgent encryption, which was done so that no one would find out, because he wanted to send the letter to Jingyang instead of Chang'an.

Li Er wanted to win him over before, and he complied, but he also knew that Li Er was a generation of talented emperors after all, and it was impossible to make friends with him, so now that he was in trouble, he had to ask Zhang Yi for help.

Although Zhang Yi was likely to think that he was attracted by Li Er and chose to stay away from Zhang Yi, in fact, he had no intention of staying away from Zhang Yi, so Xue Li deliberately added some ink in the letter to explain to Zhang Yi clearly.

In fact, Xue Li didn't want to be misunderstood by Zhang Yi, no matter what, Zhang Yi was his own benefactor, if it weren't for him, he would still be in Hedong, where he might become the main general of the Tang Army.

Moreover, Zhang Yi's knowledge of thermal weapons made Xue Li admire, no matter what, he would not break off the relationship with Zhang Yi.

The urgent letter was delivered quickly, thousands of miles away, and it only took three days for Xue Li's letter to appear in Zhang Yi's hand.

Zhang Yi saw Xue Li's letter, although he was slightly surprised, he still chose to open it.

Even if Xue Li was co-opted by Li Er, but now that he has written to himself, he must have encountered some difficulties.

He didn't care about the war during this time, so he didn't know much about the progress of the war on the front line, and now that Xue Li had written a letter, it was not impossible for him to know about the war on the front line.

Anyway, let's see it first, and it's not certain whether I can help myself.

Even if he could help, Zhang Yi would consider whether to help him, and he didn't want to experience that feeling again.

Zhang Yi opened the letter, and the first thing at the beginning was Xue Li's explanation, he explained to him what happened that day, and also said that when he arrived at Cheng Guogong's mansion, the people from Cheng Guogong's mansion told him that Zhang Yi had gone back.

The beginning of the letter was full of regret, and Xue Li once again emphasized his friendship with Zhang Yi.

Zhang Yi saw that Xue Li used so much ink to explain, and guessed that he might have been kept by Li Er for too long that day.

But no matter what, even if he really didn't come to him that day, with the current letter, he is willing to spend so much pen and ink to explain to himself, it is enough to see that he still attaches great importance to this friendship, so Zhang Yi decided to continue reading.

The second half of the letter is Xue Li's description of the current situation and the predicament facing the Tang army.

Obviously, they were a little confused about the current situation, but Li Er gave another death order.

Xue Li also indirectly revealed the opinions of other Tang army generals, if they really couldn't find any good way after a month, they could only let the Tang army attack.

Even if there were heavy casualties at that time, they had to do so in order not to disobey Li Er's orders, in order to be able to destroy Goguryeo as soon as possible, and in order to be able to end this war as soon as possible.

When he saw the end, Zhang Yi couldn't help frowning.

In order not to be punished by Li Er, let the soldiers of the Tang Dynasty desperately lose their lives to attack the city?

Moreover, according to Xue Li's description, even if they attack strongly, they may not be able to attack it, and if the Tang army loses heavy losses but cannot capture the capital of Goguryeo, then the offensive and defensive momentum of the war is likely to be reversed.

Zhang Yi didn't want to see this kind of situation, because at that time, Li Er would definitely be manic, and Li Er would be manic, and he would probably not have a good life.

He could foresee that once there was a change in the war over there, Li Er would definitely send him to the front line without saying a word, no matter what kind of name he had, even if he was tied up.

Zhang Yi doesn't want to go to that kind of ghost place to suffer in this icy and snowy time, this kind of day is nested in the quilt every day, hugging left and right, what's wrong, he has to go to that kind of place to suffer.

And when the war situation is turbulent, he is likely to be in danger of his life.

Now he no longer has a strong guard like Yue 11 to ensure his safety, if he goes to the front line and does not have a strong guard, then his safety factor will be greatly reduced.

And Zhang Yi will also worry about whether Li Er will use some small skills after the battle situation stabilizes, so that he will stay in the frontier forever and never come back.

Whether it's alive or dead, he doesn't want to come back on his own anyway, in whatever form.

Zhang Yi didn't want this to happen, so he had to help Xue Li solve this problem.

So, Zhang Yi thought about it for a while, and immediately began to write back to Xue Li.

In fact, when Zhang Yi saw the question raised by Xue Li, something immediately popped out of his head.

That kind of thing can completely solve the problem that Xue Li and they are facing now.

During the Southern Song Dynasty, the Song army developed the world's first thermal weapons - assault muskets and rockets.

This rocket is not the kind of rocket that lifts into the sky, nor is it the kind of rocket that ignites on top of an arrow.

This rocket is an explosive tied to the tail of the arrow, and when the arrow falls, the explosive will burst open, injuring the surrounding objects.

The role of the arrow here is to transport that explosive to the enemy, and then allow the explosive to hit the enemy.

Now that they can't get close to the capital of Goguryeo, they can do the same, tie a gunpowder bullet to the tail of the arrow, and shoot it at the capital of Goguryeo.

Once they did this, the capital of Goguryeo would never be able to hold it.

However, this will be a great test for the archers of the Tang Army.

First of all, the gunpowder bullet is tied to the tail of the arrow, which will greatly affect the balance of the arrow, affect the balance performance, and will also greatly affect the range of the arrow.

If the Tang archers could not handle this well, the range of their arrows would have become very short, and they would have to get closer to the capital of the Goguryeo people in order to be able to fire gunpowder bullets into the city.

And being closer means they have to pay more with their lives.

However, this method is definitely stronger than those Tang generals who want to attack with their brains in water, at least they don't have to attack, and they can use this method all the time, and they will always be consumed with the Goguryeo people.

Just imagine, if a part of the Tang army rushed to the capital of Goguryeo every day and gave them a few rounds of gunpowder shells, would those Goguryeo people be able to stand it?

If they send people to defend the city wall, those rounds of gunpowder shells are enough to make them suffer heavy losses, if they don't send people to defend it, or the defense force is insufficient, then the Tang army can take the opportunity to attack.

Either way, Zhang Yi felt that this method would work.

Moreover, now that the Tang Army has only three months left, they can't find a better way in such a short time.