Why does one federation succeed while another fails? Federations may fail for exogenous reasons: foreign invasion, collapse of its currency, or global market forces. If we focus on endogenous factors, the possible reasons for failure include diversity, where populations drift apart or clash too often, or jealousy from redistributive asymmetry, with unequal shouldering of costs or dispersion of benefits, and, of course, corruption and insurrection. While potentially significant, these problems are not particular to federalism; they threaten all democracies. There is, however, one threat that afflicts federations in particular. It is the one that perplexed and animated James Madison in the American context: opportunistic authority migration between levels of government.