The Violent Century- Lavie Tidhar

A bold experiment has mutated a small fraction of humanity. Nations race to harness the gifted, putting them to increasingly dark ends. At the dawn of global war, flashy American superheroes square off against sinister Germans and dissolute Russians. Increasingly depraved scientists conduct despicable research in the name of victory.

British agents Fogg and Oblivion, recalled to the Retirement Bureau, have kept a treacherous secret for over forty years. But all heroes must choose when to join the fray, and to whom their allegiance is owed-even for just one perfect summer's day.