Now that I think about it, there was a part of the history class in my previous life that had the sam situation, but not exactly as is. Let's try this answer

"because of the domino effect." I answered. " the empire believes that if the two states fell, the empire will most likely fall as well from demon invasion, so in order to maintain regional stability the empire decided to deploy some troops."

Similarly to the Defense Treaty, which states that if one of the two countries is attacked, the allied country will provide military assistance. There are some benefits to this pact; it contributes to regional and global stability and therefore allows prosperity to be maximized. It also deters aggression from bigger countries. The disadvantage is that they will be sacrificial countries if a war is provoked, or they will be involved in countries that the allied country opposes, as well as bailing out on you.

But this time was different. The empire was terrified of a demonic invasion and was thinking that if those nations fell, the empire might be the next frontline of the demonic invasion, which may have heavily cost them economically and militarily. As a result, they have decided to send some troops to help them maintain regional stability and security in the region.

I looked at Ludwig. He was shocked at my answer. 

"Was it wrong?"

'No, it was correct," said Ludwig. "Most of the nobles of the empire believed that the empire might possibly fall from the demonic invasion. which forced the emperor to send some troops to assist them in hindering the demonic invasion. After the end of the demonic invasion, the empire, the Burgari kingdom, and the Adonia Principality decided to create a defense pact. It is called the "Anti-demon coalition" to counter any demon invasion in the region."

The old clock then made a gong sound. It was an indication that the lesson was over for the day, and it was also lunch time.

"So that's it for the day, Luke."

after that he was preparing to leave

My father said he was a scholar at the imperial academy. Then, rather than relying on the book, it would be better to ask the person who studied at the academy.

"Wa-wait, Ludwig!" I exclaimed. "You were a scholar in the imperial academy, right?" Then do you know about magic? "

"Magic? Why?"

"I'm curious about magic, you see, and since you were a scholar at the academy, maybe you know about magic a bit or two."

"It is true that I was a scholar, but I didn't take the magic research course," he said. "However, I have general knowledge about magic."

I have a book about magic, so oh wait, if he has a general knowledge of magic, then I can confirm some of my assumptions as well by questioning him.

"OK then, can you explain it to me, and Lily, can you bring us some tea and cake as well?"

"Yes, young master." After replying to my order, she left.