Manga about a sword that only a little boy can unsheath What is that manga that is set in the 17th century about a small boy who has a sword from his deceased old man that only he can open when he's angry? The sword is longer than his body.

And he's all about revenge and the manga is often about war tactic or strategy...

Chosen people have star marks on their bodies and that boy is one of the star-marked people...
You're looking for Hero Tales aka Juushin Enbu. There's also a 26 episode anime adaptation of it.

From ANN:


  Set in a time of chaos, seven heroes of the Stars, the Hokushintenkun, rise up to lead the people. Two of the seven, the Nitenshinson, the Hagun and the Tanrou, are never supposed to exist at the same time, but fate has decided otherwise. Taito, chosen as the Hagun, now must rise up to defeat the Tanrou, as the fate of the Stars destined it to be.