Transboundary water treaties are often expected to prevent conflicts over waters from shared rivers. However, empirical evidence shows that some upstream countries continue to experience conflict after signing a water treaty. This study explains why some upstream countries experience high post-treaty transboundary water conflict levels while others do not. Departing from theories on the character of states, I argue that weaker upstream countries are more likely to experience post-treaty transboundary water conflict than stronger upstream states. This is because a weak upstream state has fewer capabilities, which creates an imbalance of power with its downstream riparian neighbor and presents a zero-sum game condition. As a result, the upstream state is more likely to experience a high level of conflict after signing an agreement. The hypothesis is tested on two transboundary river cases, the Helmand River Basin and the Indus River Basin, using a structured, focused comparison method. The data is collected through secondary sources, including books, journals, news articles, and reports, government records. The results of the study mainly support the theoretical arguments. It shows a significant relationship between the character of the upstream state and the level of post-treaty transboundary water conflict in the upstream state.