Who where the military leaders during the period of the lord-vassal system?

The military leaders during the period of the lord-vassel system were "shoguns." The Shogun was so powerful that he was the real power and directed the politics of the empire.

could you use this to with a topic?

the key difference between a democracy and a republic lies in the limits placed on government by the law, which has implications for minority rights. both forms of government tend to use a representational system — i.e., citizens vote to elect politicians to represent their interests and form the government.