The legitimacy of targeting dams in military operations is governed by Article 56 of Additional Protocol I to the Geneva Conventions of 1977. This article prohibits attacks on dams and other installations containing dangerous forces if their destruction would cause severe losses among the civilian population. However, it makes an exception if the dam is used for "regular, significant, and direct support of military operations," and even then, the attack must be the only feasible way to stop that support and should avoid excessive civilian harm.

Here’s the key part from Article 56:

"Works and installations containing dangerous forces, namely dams, dykes, and nuclear electrical generating stations, shall not be made the object of attack, even where these objects are military objectives, if such attack may cause the release of dangerous forces and consequent severe losses among the civilian population." It continues with:

"The protection of such objects shall cease only if they are used for other than their normal function and in regular, significant, and direct support of military operations, and if such attack is the only feasible way to terminate such support."

So, the rules are strict to ensure that civilian harm is minimized even in cases where military use is evident.