The feeling of incorrectness, in my opinion, stems from the connotation of "accordingly." There are other usage irregularities in this sentence, but to focus on the one in question, I would suggest taking note of the subtle variations in meaning of some possible synonyms or non-synonymous alternatives to "accordingly." First and foremost, "accordingly" itself implies that the linkage between the two elements is a matter of accord, which is a conscious, usually intentional agreement of thought. When we tease out the meaning this way, we can see that it doesn't suit the given usage very well. The army is not in accord with the empire.
Now consider these options:
Correspondingly: This would be a good option, I think, since it suggests a concordance of size that does not require a literal equivalence.
Equivalently: This falters in that the correspondence is implied to be very much the same within the realm of some parameter of size, such as length, number, mass, or anything else you choose to define the two items, but it isn't really possible to create this kind of concordance between two such disparate entities as an empire and an army.
Appropriately: This works in that it generically allows for the rightness of the concordance, but I would not choose this alternative unless I wanted to emphasize the moral, political, or philosophical dimension of the choice inherent in the creation of the army.
There are other options, but this should be enough to point out that it is essential to consider the subtle connotations of any word you might choose to fill this space.