Why is there a を there? Is it a word or does it have a specific structure?
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This を is still an object marker, but the corresponding verb is omitted because it can be inferred from the context. This happens very often in slogans, headlines and lyrics. You can see some examples here:
In your case, depending on his purpose and the context, I guess the full sentence should be something like この国を変える ("change this country"), この国を良くする ("make this country better") or even この国を拙者のものにする ("make this country mine"), but I may be wrong. You should be able to tell from the context if you understand the story well enough.