The Romans are an example of a nation that had a theocratic government system at one time. A theocracy is a nation that is under one religion or god; this has its advantages and disadvantages. Several advantages to this system would be that people would all have the same ideals (in theory). The same morals and values would be put into play and people would all have the same laws to abide by. Being under the same religion, people would be under the same church authority and the government would be accepting of church practices.
Some disadvantages to this would be a lack of religious freedom. Yes, people would be able to practice religion, but it would have to be the state religion. Therefore, if you did not agree with the religion, it did not matter. Also, this gave the church and the government a lot of power which could easily be abused. Almost always, a nation is made up of a very diverse group of people. I can imagine that if there was only one religion in a society, there would be much rebellion against it.
It's interesting to look at America and see the diverstiy of religion within my own country. I know as a Christian, my faith is very important to me and I am grateful to have the ability to practice my faith without being persecuted becuase of it. In my opinion, religion should not be used as a form of control; it seems like that is what ends up happening under a theocratic government.