ATHENS, Greece—It’s only the size of a shoebox, carved with the broken-off foot of an ancient Greek goddess. But Greece hopes the 2,500-year-old marble fragment, which arrived Monday on loan from an Italian museum, may help resolve one of the world’s thorniest cultural heritage disputes and lead to the reunification in Athens of all surviving Parthenon Sculptures—many of which are in the British Museum. Greek Prime Minister Kyriakos Mitsotakis said the Sicilian museum’s gesture “opens the way, I believe, for other museums to be able to move in a similar direction.” The fragment was part of a 520-foot frieze that ran around the outer walls of the Parthenon Temple on the Acropolis, dedicated to Athena, goddess of wisdom. Much was lost in a 17th-century bombardment, and about half the remaining works were removed in the early 19th century by a British diplomat, Lord Elgin. They ended up in the British …