Let's rewind time a bit.

The founding of the Snakes Kingdom was reported to the Empire by a group of merchants posing as merchants who were infiltrated by Zabeen's intelligence service.

The Xavian Empire was desperate in its own way to obtain as much food as possible.

'That's why there is a possibility that the Mental Kingdom will split up and the Mental Kingdom and the Snakes Kingdom will enter into war. It is rumored that some nobles in the Mental Kingdom are advising the king to attack the Snakes Kingdom.
He said to his subordinate who reported to the Emperor,

If that's the case, we can either attack when the war between the Mental Kingdom and the Snakes Kingdom starts, or when the war is over and we're exhausted. The Non-Aggression Pact was signed with the Mental Kingdom, not the Snakes Kingdom, so we can attack at any time. Even if the Mental Kingdom wins, they're exhausted and won't have recovered their national strength when the Non-Aggression Pact expires! This is our chance to take back the mines near the border. Yeah, it's a wheat-growing area, so we'd better take it after the harvest is over!
The Emperor said with a laugh.

'That's right. It would be better to take the food. It's better to avoid the farmland and fight a war to capture it, so we can expect a good harvest later on.

'We can't afford to make the same mistake we made last time,' he said.
The Emperor acknowledged the failure of the previous war.

And so, in order to keep an eye on the two countries, the Xavian Empire has decided to gather information from merchants with money in their hands.
Of course, he sends his own agents.
But there was no good information coming from the many merchants. Because,

'It would be more profitable to be a Snakes than a Xavian.

'The tax rate is much lower in the Snakes.

'We might have a chance to stock up on Snakes' liquor.
The merchants of Zabeen did not like the high tax rate of Zabeen.

It would have been a miscalculation for the agents who relied on the merchants. But for the agents who did not move around much on their own, it was a lifesaver. Because the hard-working agents had already disappeared from the world.

'How many today?
Ayn asked his men,
'Eight, sir.
'Eight,' comes the reply.

'You are wasting your time. It's impossible to extract information from our country. As long as we're here.
Ain said calmly.

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'Are there a lot of big wagons coming and going at the Snakes' fort?
After hearing the information from some merchants, the Xavian Imperial Command reported it to the Emperor. The Emperor heard it and asked back as follows.

'Yes, it seems that wagons are entering the fort every day.
'Yes, wagons seem to be entering the fort every day,' said one of his men,

'Isn't that food for the spring? It's almost winter. They're probably bringing in the wheat harvest. You haven't heard anything from your agents, have you?

'Yes, sir, nothing of note.

'Then there is nothing to be concerned about. If they can bring in a large number of horses, they might attack us. Well, put a few more men on the east side. If Mental and Snakes get into a fight, they might attack.
With those words, the matter was put to rest.

Emperor Zabeen, who was in a castle far away from the Snake Kingdom, did not know.
It's the merchant's fault for not reporting the size of the wagon properly, but it was a wagon that could hold several horses.
And that a squad of vipers can run all day and night without horses.