In a speech to the U.S. Congress on Tuesday, King Charles III said the U.S.-U.K. alliance "cannot rest on past achievements." The call on the allies to continue work on their alliance comes as differences over the Iran war tests the relationship, perhaps the lowest point in the so-called "special relationship" since the 1956 Suez Crisis, when the U.S. opposed Britain's attempt to seize the Suez Canal.