Those wars were conditions resulting from the large changes of the whole cosmos, and you were only an element of the conditions. Without the human activities under the influence of cosmic changes, such conditions could not have been brought to human society, nor would there have been the so-called cosmic changes. Such events change with great cosmic changes, and therefore they are not totally your responsibilities. The karma we talk about here refers to what one has accumulated because of wrong doing committed out of selfish motivations, personal interests, or what is related to oneself. It is no fault of yours as long as it is related to large-scale changes of the social situation, big changes of the whole space.
Killing brings about enormous karma.
This specific issue is not my concern. I am teaching the Law to practitioners, not casually telling ordinary people how to lead their lives. You should act according to the requirements of the Great Law to deal with those specific issues. Do what you feel is appropriate. Ordinary people will do whatever they want to do, which is the business concerning ordinary people.