In order for a democracy to resist the slide into tyranny, a division of powers is needed. When power is held by any one organ of government it can become corrupt. Installing two or more forms of government within a state creates a balance of powers. National government is checked by local state government and local government is also checked by the central government. Centralized states on the other hand, inevitably become more rigid, elitist, and even tyrannical. Under a federal system, in the event that a central government falls to a dictatorial leader, the regional government can protect its citizens. Legislation which is draconian or unpopular with the majority can be circumvented by the local legislature.  If policing and the judiciary have been partially devolved to local bodies, they can serve as useful preventative measure in a worst-case scenario. It would be difficult for a dictator to establish power from the center when they have little control over police nationally. Federalism is one way of protecting democracy. It is a useful way of separating powers, it is one way of creating a system of checks and balances.
Federal governments are not immune to tyranny, although they may slow its inception. The most prominent example of a federal state turning to fascism is Weimar Germany. Upon getting elected Hitler simply revoked the powers of other sovereign bodies.  Sovereign states typically retain the right to revoke the powers of its constituent bodies at will, especially in emergencies. When states overstep on political issues which are unpopular with the central government, their desires are frequently ignored. Many political theorists believe that cultural norms and moral values are the only real barrier to dictatorship.  The central government’s sway over the national army is often the trump card which allows them to take power. Even if federalism allows for checks on the military it cannot stop fascists from raising their own militia to override the established order. History shows that federalism does not make a democracy immune to dictatorship. Federal powers can be revoked by the central government, who often hold sway over the military. Powerful and charismatic dictators often find ways to raise loyal forces that bypass the separation of powers. Local government can also be tyrannical or unjust.
[P1] The division of powers can safeguard against one group becoming too powerful in a democracy [P2] In the event of a dictator coming to power, regions can resist with their own powers [C] Federalism is an important way of protecting democracy
Rejecting the premises