From wikipedia:
Pronunciation of lieutenant is generally split between the forms lef-tenant (/lɛfˈtɛnənt/) and loo-tenant (/ljuːˈtɛnənt/ or /luːˈtɛnənt/ ( listen)), with the former generally associated with the United Kingdom, Ireland and Commonwealth countries, and the latter generally associated with the United States.[1] The earlier history of the pronunciation is unclear; Middle English spellings suggest that the /l(j)uː-/ and /lɛf-/ pronunciations existed even then.[2] The rare Old French variant spelling luef for Modern French lieu ('place') supports the suggestion that a final [w] of the Old French word was in certain environments perceived as an [f].[2]
In Royal Naval tradition — and other English-speaking navies outside the United States — the intermediate pronunciation /ləˈtɛnənt/ was preserved. This is not recognized as current by the OED, however, and by 1954 the Royal Canadian Navy, at least, regarded it as "obsolescent" even while regarding "the army's 'LEF-tenant'" to be "a corruption of the worst sort".[3]