Naval treaties relied on mutual restraint

Who interpreted and enforced the interwar naval treaties?

The participating governments did. With no independent arbiter, enforcement depended on mutual restraint, tolerated limits on cheating, and ultimately the willingness to abandon the treaty or go to war.

The answer begins with the practical question of who interpreted treaty language such as Article 19's longitude boundary. Each government made its own case because no independent arbiter possessed routine enforcement power. Navies also exploited the wording in different ways, including definitions that favored their existing design practices. The choices were to tolerate limited evasions, admit that the regime had collapsed into an uncontrolled building race, or treat a violation as serious enough to justify war. That structure made mutual restraint and political judgment more important than strict legal policing. Treaty compliance could remain useful even when every participant suspected some degree of manipulation.