## Facts

- Hungary and Czechoslovakia concluded a 1977 treaty to construct and operate a joint Danube system of locks at Gabčíkovo (then Czechoslovakia) and Nagymaros (Hungary) as a single operational system serving energy, navigation, and related aims.
- The treaty included commitments to protect nature and prevent impairment of Danube water quality during construction and operation.
- Work began in 1978; in the late 1980s Hungary slowed, then suspended, and ultimately abandoned works assigned to it, citing economic constraints and environmental concerns.
- Czechoslovakia pursued an alternative “provisional solution” (Variant C) involving unilateral construction and diversion of most of the Danube onto its territory to enable independent operation.
- On May 19, 1992, Hungary notified termination of the 1977 treaty and related instruments.
- After Czechoslovakia dissolved, Slovakia became the successor in interest for rights and obligations relating to the project.
- The parties submitted the dispute to the International Court of Justice by Special Agreement (1993), requesting rulings on Hungary’s suspension/abandonment, Czechoslovakia’s Variant C, the legal effect of Hungary’s purported termination, and resulting legal consequences.

## Issues

1. Whether Hungary lawfully suspended and abandoned treaty performance based on necessity, impossibility of performance, fundamental change of circumstances, material breach, or environmental developments.
2. Whether Czechoslovakia (and later Slovakia) could lawfully construct and unilaterally operate Variant C, including diversion of the Danube.
3. Whether Hungary’s 1992 notice lawfully terminated the 1977 treaty under treaty law grounds for termination or suspension.
4. What legal consequences and remedies followed from any internationally wrongful acts, including obligations of cooperation, adjustment of performance, and compensation.

## Decision

- The Court held Hungary was not entitled to suspend and subsequently abandon, in 1989, the works assigned to it under the 1977 treaty and related instruments.
- The Court held Czechoslovakia was entitled to proceed in November 1991 with the provisional solution (Variant C) in response to Hungary’s prior non-performance, but was not entitled to put Variant C into operation unilaterally from October 1992.
- The Court held Hungary’s May 19, 1992 notification did not terminate the 1977 treaty; the treaty and related instruments remained in force.
- The Court held Slovakia became a party to the 1977 treaty as successor regarding the project.
- The Court directed that the parties must negotiate in good faith in light of prevailing conditions, establish a joint operational regime absent agreement otherwise, settle accounts under the treaty framework, and compensate each other for damage caused by wrongful conduct.

## Legal Principles

- Necessity is an exceptional defense requiring a grave and imminent peril, an act that is the only means to safeguard an essential interest, and no substantial contribution by the invoking state to the situation; environmental protection may qualify as an essential interest, but the threshold remains strict.
- Termination or suspension for material breach, supervening impossibility, or fundamental change of circumstances is narrowly confined; treaty stability requires a high threshold and restrictive application of these grounds.
- A countermeasure must be proportionate and cannot lawfully result in unilateral appropriation or control of a shared natural resource; unilateral operation that deprives a co-riparian of its equitable share may exceed permissible countermeasures.
- States sharing an international watercourse must respect equitable and reasonable use; unilateral diversion that effectively excludes the other state from its share is incompatible with that obligation.
- Environmental obligations within a development treaty remain relevant to its interpretation and performance; implementation must take account of contemporary environmental considerations and the need to reconcile development and environmental protection (often expressed as sustainable development).
- Where both parties have committed wrongful acts, remedies may include reciprocal compensation, accounting, and a duty to cooperate through good-faith negotiations to achieve treaty objectives under present conditions.

## Conclusion

The Court maintained the continuing force of the 1977 treaty, found Hungary’s suspension/abandonment and Slovakia’s unilateral operation of Variant C unlawful, limited permissible responsive measures by proportionality and shared-resource constraints, and required good-faith cooperation, joint management arrangements, and compensation to restore treaty-based performance under current environmental conditions.


Gabčíkovo-Nagymaros Project (Hung./Slovk.), 1997 I.C.J. 7 (Sept. 25)