Landmark judgment of the International Court of Justice in the case concerning the Gabcikovo-Nagymaros Project (Hungary v Slovakia), decided on 25 September 1997. Arose from a 1977 treaty between Hungary and Czechoslovakia to construct a system of locks on the Danube River. Hungary suspended and later abandoned works citing environmental concerns; Slovakia (successor to Czechoslovakia) proceeded unilaterally with 'Variant C' diverting the Danube. The ICJ held that: Hungary was not entitled to suspend/abandon works; Slovakia was not entitled to put Variant C into operation; both parties had breached their obligations. Established key precedents in transboundary water law including: the continuing obligation to negotiate in good faith, the relevance of environmental concerns in treaty interpretation, the concept of 'approximate application' of treaties, and the principle that a shared resource creates a community of interest. Frequently cited in subsequent transboundary water disputes.