Where in the Constitution does it guarantee that the national government will not usurp state powers?
That can be found in the tenth amendment of the Constitution.
Although the 10th amendment was not originally a part of the Constitution, it's an amendment so it is part of the constitution. The amendment states "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people," meaning that any powers that the national government does not have that are not restricted from the states are reserved for the states. That's why you see states with their own police forces and not a national police system (though the national government does have some police forces which only enforce national laws).