Why did Britain provide a defense guarantee when Belize became independent in 1981?

Britain provided a defense guarantee to Belize upon its independence in 1981 primarily due to the persistent and "unrelenting Guatemalan hostility" regarding territorial claims over the region.12

For decades, Guatemala challenged the British occupation of the territory, asserting that it had inherited Spanish interests in the area. Although a treaty in 1859 had attempted to define boundaries, the dispute became intractable after 1940 when Guatemala declared the treaty null and void, claiming that required infrastructure projects had never been developed.34

Because this territorial dispute remained unresolved at the time of Belize's independence, the British defense guarantee was essential to ensure the security of the newly sovereign state against potential aggression. This military presence remained in place until 1994, at which point the responsibility for border security was transferred to the Belize Defence Force.12