I don't see how this is particularly unrealistic.
Obviously I expect something in return for my troubles! I just wanted to help you so you would owe me big in future.
Also, what is wrong with helping you defeat the rising factions? They may take a power, find a grand mineral! For all we know, they could dominate the world!
Paper invented by Aemilius.
Testing something unrelated:

If you do not understand why it is unrealistic, let me explain the ancient era.
Isolationism was key. If you had a neighbor, they were probably a bitter rival an enemy competing constantly for more land and resources. There is absolutely no example of two nations just being randomly friendly giving eachother support in a war. What happens if some dumb nation decides to do this:
Neighboring nations are almost without exception hostile.
Neighboring nations see you army going far away to distant lands for no good reason.
Neighboring nations see the massive toll on your workforce that equipping so many people for such an expedition has cost.
You are weak. Your nation is swiftly conquered.
That is really how the situation would go.
A nation with the distance that Nexia has with most of these other nations makes it unreasonable to send soldiers. Avalonia is somewhat far (considering going around the world) but Aemilius is pretty ridiculous.
There was pretty much just the Roman Empire in the ancient times we're talking about, with Egypt. We can't really know how it would play out with this many, Nexus.
These was china, egypt, mesopotamia, germanic barbarians, african tribesmen, the islamic (moor) empire, mongols, greek empire, turkish empire, persians, and all of them had one of the following traits:
- Unaware of other nations (as a general rule)
- At war with nearby nations
- More distant nations had some trade, but it was often raided by barbarians and pirates.
Almost all nations were at least hostile with eachother.All foreign relations was entirely consisted of war
In my opinion, it is silly to jump so fast to distant things that do not concern you, and not really possible in the ancient empire. I do not expect everyone to be mirroring the ultra-hostile nature of true ancient times, but this magical friendship thing everyone has going on is poop.