It is never revealed why Dahan went to great lengths to capture Maya. They must have realized that kidnapping her would have caused an international incident, but they did so regardless. The only logical reason I can think of is that they believed the benefits of their experiments on her would outweigh any negative consequences. However, what were these experiments? All we know is that they resulted in her being unable to bear any children. So, what if they were researching magical genetics for some purpose? What are your thoughts?
While it may have been the Third World War at the time, Dahan was informally allied with Japan against the Great Asian Alliance. So, by kidnapping Maya they were risking losing that alliance and isolating themselves. Why throw away your only ally in wartime just to perform experiments on a girl?
i don't think so, they may have just wanted a test subject to experiement on her and do immoral things. They might've been unsure about the power of youtsuba's, kidnapping might've been purely random. And as youtsuba's were not in govt they might've been sure that the alliance will not be affected.
and can you please specify where it's mentioned that Dahan was in Alliance with Japan.
From the page titled "Dahan" on this wiki. It is explained in volume 8 that though Japan and Dahan were not formal allies, they shared an enemy in the Great Asian Union and cooperated militarily. Also, there is no way they would've been unsure of the power of the Yotsuba as they were part of the Ten Master Clans even back then. While the master clans do not take a direct part of governance, they still regulate the affairs of the magical community. Furthermore, the master clans were originally created using genetic manipulation from the different Magical Institutes. So, the Japanese would have seen the experiments on Maya as an attempt by Dahan to learn the secrets of their Magicians, and this would be a direct threat to their interests. Finally, why would a Dahan (which was at war) waste resources to randomly capture and experiment on a girl just to do "immoral things?"
It wasnt about immoral things per say. It was about acquiring a powerful weapon in their eyes. Powerful Magicians are worth 40 to 100 people on their own. Acquiring a Yotsuba's gene's was likely worth any price including alienating an ally.
1. It's stated that "Kudou Retsu" is one of the most powerful magician at time and Maya is his (direct) student. so, they may thought Maya may know some of his magics.
2. whether or not, Maya's Meteor Stream is One of the Strongest Magic which can't be blocked by Planax either. This may lead them to kidnap Maya and experimented on her.
so, they may get hold of "Meteor Stream".
3. It is known that the Dahan (example: Zhou Gongjin) also has Mental Interference Magic (Example:Ghost Walker),so there is a lot of chance that they want analysis Japan 's(Yotsuba) MI magic or their genes as well.
so, they chosen Yotsuba's Kid.
4. At that time, Yotsuba isn't that much feared (publically), and Clan Head's (MI Magic user's) daughter is Maya.
These are my thought, correct me if I am wrong;
I hope, it will help you to think in a wider prepespective.
While I'm not sure the author actually had any reason to do this at the time he wrote it, a reason could have been created down the line.
My top of the list is that someone from Japan (either from another major family or the government) paid them to do it to create a conflict between the Saegusa and the Yotsuba.
These two families were already very strong at the time--especially the Yotsuba, and looking to grow even stronger with Miya and Maya growing up--and a marriage alliance between the two would pretty much mean their domination of Japan.
The only reason the 10 clans are relatively stable is because Yotsuba and Saegusa hate each other now. Imagine the power imbalance if they were as close as family.