Break Blade

The series is centered around Rygart Arrow, an "un-sorcerer" born into a world of people who can use magic. These people can make quartz do many things, but Rygart is one of the few exceptions; he cannot use quartz, making many aspects of life difficult for him. Yet despite this, at Assam military school, he manages to become best friends with Hodr and Sigyn, the future King and Queen of the Krisna Kingdom and Zess, the younger brother of the Secretary of War of the Athens Commonwealth.

Years later, Rygart is reunited with Hodr and Sigyn at the Capital city of Krisna where he learns the Athens Commonwealth has just declared war on Krisna. But it is at the capital that Rygart learns that his un-sorcerer ability allows him to pilot an ancient, effective, mecha that cannot be piloted by magic users.

Rygart later learns that Zess is leading a mecha strike force to the capital which forces him to choose sides between his friends, Hodr or Zess.

Source: Wikipedia