The kingdom went into decline from the mid-16th century CE when the Portuguese, put off by the interference of Kongo's regulations on trade, moved their interests further south to the region of Ndongo. The latter kingdom had already defeated a Kongo army in 1556 CE.
It's A) the US must be a leader in innovation in order to help steer its course.
When he went to Milan, the Duke Sforza appointed him to be a military engineer where he had to design weapons and technological advancements that would help the duke gain warfare supremacy against enemies. He also built buildings and defenses, not just offensive weapons.