Why does one federation succeed while another fails? Federations mayfail for exogenous reasons: foreign invasion, collapse of itscurrency, or global market forces. If we focus on endogenousfactors, the possible reasons for failure include diversity, wherepopulations drift apart or clash too often, or jealousy fromredistributive asymmetry, with unequal shouldering of costs ordispersion of benefits, and, of course, corruption and insurrection.While potentially significant, these problems are not particular tofederalism; they threaten all democracies. There is, however, onethreat that afflicts federations in particular. It is the one thatperplexed and animated James Madison in the American context:opportunistic authority migration between levels of government.