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This paper explores two mechanisms through which a sovereign default can disrupt the domestic economy via its banking system. First, a sovereign default creates a negative balance-sheet effect on banks, which reduces their ability to raise funds and prevents the flow of resources to productive investments. Second, default undermines internal liquidity as banks replace government securities with less productive investments. I quantify the model using Argentinean data and find that these two mechanisms can generate a deep and persistent fall in output post-default, which accounts for the government's commitment necessary to explain observed levels of external public debt. The balance-sheet effect is more important because it generates a larger output cost of default and a stronger ex-ante commitment for the government. Post-default bailouts of the banking system, although desirable ex-post, are welfare reducing ex-ante since they weaken government's commitment. Imposing a minimum public debt requirement on banks is welfare improving as it enhances commitment by increasing the output cost of default.