I actually don't believe the federal government has very little power. The Supremacy Calause, the Civil War, erosions of the 10th amendment (the US code is intense!), having a court called the "Supreme" court, regulating interstate commerce, adjudicating disputes between states, raising an army, and of course taxing citizens in any state should provide ample support to the federal government's power. And yes, they also control individual states by proxy using fund distributions as a carrot (road standards, national speed limit). They control immigration into the US; no individual state has the power to invite a foreign citizen into a US state.