 If you liked Kingdom |  ...then you might like Shingeki no Kyojin |
Both are two of the best military manga I've ever read. Both have a typical shounen protagonists facing a morally grey world. Both have excellent character development. There are also interesting politics, even if Kingdom arguably does it better.
Two manga with lots of wars and military tactics to take down the opposing factions. Don't get too attached to anyone, because they might die at any moment. They both have an excellent way to explain how the smart generals can make decisions that ultimately have an enormous impact, from either the side of 'good' and 'evil', which tends to get blurred over time. *As with any war-related manga, gore scenes are very common.