If you're looking for manga similar to Horizon in the Middle of Nowhere II, you might like these titles.
Half a century ago, the human race was attacked by an enemy from the sea known as the AUS. When Reiji Shiroyama was little, he was unable to protect his sister from the AUS because the gun he tried firing jammed. Since then, Reiji refuses to trust guns and only trusts his fists.
The science-fiction war action story revolves around Objects — the powerful, massive weapons that change the course of warfare — and Elite Object pilots. An odd Elite girl named Milinda meets Quenser, a student who aims to become a Object mechanic, with the soldier-in-training Heivia on a snowy battlefield.
Takes place in the world after the "end" and follows machine soldier Lear, who has been following the "Last Order" and continuing an endless war for over 100 years.
As the countries of the world continue to fight each other over seemingly trivial matters, the human race continues to experiment with solar and atomic bombs. For many years, the Nation of Star has conducted their nuclear research on the secluded Horseshoe Island and, as a result, the creatures living there have begun to rapidly evolve. One such species, known as the Fumoon, have recently emerged and are able to harness the power of gravity to send not just people but even warships flying! Though with the world at war, and a new race of creatures threatening the human race, Earth is in further peril as a dark gaseous cloud is rapidly heading for the planet and will extinguish all life when it arrives...
A retired thief comes out of retirement to stop a scientist – who was his last victim – from deploying a bomb that will destroy all the air on Earth. The other tale is an anti-war parable set in World War III, about two bomber pilots who become friends after they duel with each other and both crash.
The remnants of a defeated Empire prepare to strike back at an invading army.