From what I read in the book, it seems that Sigmar was (as befits a god) as brilliant as he was dangerous. He learned at a young age the importance of outwitting your foes, in addition to overpowering them.
The first two clans to fall under his service actually united to conquer the Unberogens, but Sigmar (having rescued the High King by this point) had a brilliant plan. They both demanded tribute from him, so Sigmar gave them dwarven weapons and armor, revealing that his men were better armored, better armed, better trained than BOTH regions, as well as having dwarf allies that could aid them in war. It left both tribes too frightened to dare to challenge him, and left Sigmar essentially in the position of giving THEM orders without a single drop of blood shed.
Really great read.