Why was Belize the last British colony on the American mainland?

Belize, formerly known as British Honduras, was the last British colony on the American mainland primarily due to persistent, unresolved territorial claims by its neighbor, Guatemala.1

While other British territories in the region had moved toward independence, Belize’s path was uniquely impeded by this long-standing dispute. Guatemala challenged the British occupation, asserting that it had inherited Spanish interests in the area. Although a treaty in 1859 attempted to define boundaries, the relationship became intractable after 1940 when Guatemala declared the treaty null and void, claiming that Britain had failed to fulfill a requirement to establish communication between the two territories.2134

This "irredentist" claim created a security concern that necessitated a prolonged British presence to ensure the territory's defense. Belize eventually achieved independence on September 21, 1981, supported by a British defense guarantee, and continued to pursue international recognition and a resolution to the border dispute through the United Nations and the International Court of Justice.21