In 1981 Robert Axelrod famously hypothesized that international cooperation might evolve through direct reciprocity. In our new article in Science Advances, we present new evidence for that hypothesis. Using a new method and dataset, we're able to detect direct reciprocity in a relatively large number of country pairs. Reciprocating country pairs exhibited higher levels of stable cooperation, and were more likely to punish (yet quickly forgive) instances of non-cooperation, in an effort to maintain cooperation. By contrast, countries without reciprocal strategies were more likely to exploit each other's cooperation and abandon mutual agreements. The results indicate that policymakers of powerful countries—who may be tempted to take sweeping, non-cooperative action in areas like trade or environment—should consider the long-term negative effect of non-reciprocal tactics.