From AnimeNewsNetwork:
The story is set in the near future not long from now, when humanity has completed an orbital elevator and begun developing the outer planets in earnest. An enigmatic extraterrestrial enemy known as "Obliquus" has set its sights on the solar system, and humanity enters a state of war with the first aliens it has encountered.

To counter the enemy's overwhelming technological superiority, the Khronos Foundation has assembled the descendants of every country's heroes from Japan's Musashi Miyamoto and Ryōma Sakamoto to France's Napoleon and Joan of Arc. The foundation then fuses the descendants with "Gear" battle suits as special biological weapons, thus giving birth to "Khronos Gear."