When the US Constitution was written, the States feared a strong Federal Government. Because of the three branches system the US Constitution created a new system that revolutionized Constitutions around the world: the Checks and Balances system.
The Checks and Balances system is a system that the US Constitution created to give powers to each of those three branches to limit the power of the other branch. Each branch has typical and atypical powers, and those atypical powers are the ones that are used to limit the typical powers of other branches.
Because of that, if any of the three branches exceeds its authority the other branches have the power to stop this power and return the thing to status quo ante.
Example of this atypical powers is: The Power of Veto the President has, the Impeachment Power of the Congress and Power the judicial branch has to declare a law unconstitutional.
As you can notice, the Constitution created a system that autoregulates itself so that power and democracy are stable.
