Traditional models of sovereign debt assume that governments seek to maximize the long terminterests of their countries.We assume instead that governments borrow and default according to their own political interests. In particular they often have limited horizons and are reluctant to default strategically. This allows us to define a maximum sustainable debt to GDP ratio, and compute it as a function of the countrys fundamentals. We find that maximum sustainable debt varies a lot across countries, consistent with the notion of country specific debt (in)tolerance. Actual debt ratios are below their maximum sustainable levels, as governments seeking further terms in office fear debt-induced default that may jeopardize their prospects for reelection. The difference between actual and maximum sustainable debt ratios creates a "margin of safety" that allows governments to increase debt if necessary with little corresponding increase in default risk. The probability of default climbs precipitously once the margin of safety has been exhausted.