Fullmetal Alchemist, Tripeace Recommendations
The main characters have a "dark hidden past" (some more than others), in both mangas (it doesn't end up explained in Tripeace though, since it's only translated up to chapter 30). The "mood" given off by war is very similar in these two mangas. Both mangas have a dominating militaristic country and many conspiracies. They both portray a cold world and the crimes against humanity an invading country can inflict towards civilians. Despite the serious parts, the mangas are also very funny and have humorous moments.
At first glance, Tripeace seems like a copycat of FMA. The shounen art style is similar, and lots of the characters in Tri look like Ed from FMA. The worlds look similar (steampunk along with the countryside); soldiers, an all-purpose healing medicine, death, war, tragedy, comedy. Nana and Ed have similar personalities.