According to this story, Odysseus learned from the prophet Calchas that the Achaeans would be unable to capture Troy without Achilles' aid.
ɐkˈoːɹdɪŋ tə ðɪs stˈoːɹi, ɑːdˈɪsɪəs lˈɜːnd fɹʌmðə pɹˈɑːfɪt kˈæltʃəz ðætðɪ ɐtʃˈiənz wʊd biː ʌnˈeɪbəl tə kˈæptʃɚ tɹˈɔɪ wɪðˌaʊt ɐkˈɪliːz ˈeɪd.