Proportionality
The requirement that a legal measure must be justified by what it is trying to achieve — and no broader than that.
Proportionality is the international law principle requiring that any measure — a sanction, an investigative step, a legal request — must be appropriate in scope relative to the legitimate objective it pursues. A measure that imposes a burden greater than the objective requires is disproportionate, and therefore legally vulnerable to challenge or refusal.
In mutual legal assistance, proportionality is an assessment criterion applied by the requested state before executing a request. The assistance sought must be commensurate with the seriousness of the offence and the centrality of the evidence to the proceedings. A sweeping production order in a minor regulatory matter may not satisfy it. A targeted request for specific banking records in a billion-dollar fraud will generally pass. The assessment is legitimate, necessary, and — in the hands of a requested state with its own reasons for delay — extensible.
The principle operates similarly in international sanctions and the use of force, where it requires that coercive measures not exceed what the triggering conduct warrants. In every context, the logic is the same: the means must fit the end. In grand corruption cases, determining what fits takes time. That time is rarely neutral.
Sources: ICJ, Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States) [1986] ICJ Rep 14; Charter of the United Nations, Article 2(4); FATF Glossary and AML/CFT Library (aml-cft.net).