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In many real-world social dilemmas the surplus from insider cooperation can be stolen by outsiders. We present experimental evidence of cooperation to create and defend surplus under positive and negative externalities. A decentralized group of insiders created surplus by providing a public good (PG) or managing a common-pool resource (CPR), and used sanctions to deter outsiders from stealing the surplus created by these activities. Our theory predicts that theft would have the same effect on cooperation across settings. Instead, we found that an outside threat had a divergent effect on insider behavior. Surplus creation was significantly higher in the CPR setting, while surplus defense was significantly higher in the PG setting. However, sanctions did not significantly impact outsider behavior in either setting. Our results underscore the need for effective coordination mechanisms to help groups coordinate decisions when facing a shared threat.