Good to hear a new writer is taking up the pen to do manga!
It truly is a glorious storytelling method!! (I'm too lazy to do novels.)
Here's a few tips you should probably run your story through.
Does it sound uninteresting and unoriginal if you explain it in 2 sentences?
The reason I ask this, is because a major mistake that new writers do in an attempt to make their series out to be different from all the other series they've read, is try to overcomplicate things. The more complicated a story, the more original it'll seem when you pitch it, but the more frustrating and annoying it is to write and read.
That's not to say you can't have big and complicated series, but you should do so over time. If first chapter already introduces warfare between major empires and each empires quirk, then it's most likely not going to be very interesting to read for the reader.
Take Code Geass, for an example. Code Geass starts out by introducing us to these different characters, and making us like the different types of people, and then it gives us a big nice summary of the world.
"Britannia has conquered 60% of the world, almost the entirety of the west, and large parts of Asia and the Middle East."
Now, the story introduces us to this, so it sets the tone of a big series, but it also doesn't do the mistake of trying to cram too much information down our throats right away. We learn that Japan is referred to as Zone 11 and Japanese people are "Elevens", as they are denied their Japanese title.
And if you dig even deeper into the story, you see that the story is essentially alternate history where Napoleon won England, and forced the British nobility to their American colonies, where they took more attention to warfare and conquering.