I agree that invasion increases the death rate locally. But over time the invasion may have saved lives. E.G., if Geremany was invaded before it gained power for WW2 then the holocaust may not have happened and perhaps (but not certainly) more lives would have been saved in the long run (but perhaps that would cause other even worse events to unfold).
Do you understand how compound interest works? Do you understand why putting more money in an account early is exponentially better than spreading out deposits into that account over a long time, even if you put more money in for each deposit?