Why did the Federalist believe that the central government could not become to powerful?

The Federalist Party was worried that the states would seek too much power and would ultimately decide to become their own independent nations. There were many examples in Europe of independent small states; the Italian city-states and those of the Holy Roman Empire are the most famous examples of independent small states. This division would lead to financial insolvency as the states would bicker over who would pay off the debt from the Revolutionary War. There was also the worry that states would seek international alliances and attack other states. This happened with the Italian city-states, and, as a result, they were dominated from time to time by Spain and France. The Federalists believed that a national army would become a source of unity and would also provide national defense in case of attack.
The compromise finally agreed upon by the Federalists and the Anti-Federalists created the Tenth Amendment in the Bill of Rights which ensured that the states would always have some power within the federal system. This allowed more political matters to stay with the people in the states and left national matters up to the federal government. The Tenth Amendment served as a brake against overpowering federal control.