ɪn nˈaɪntiːnhˈʌndɹɪd θˈɜːɾi tˈuː, ɪmpɹˈɪzənd ˌɔn tɹˈʌmptˌʌp tʃˈɑːɹdʒᵻz ʌv ʌnlˈɔːfəl ɐsˈɛmbli, hˈɔːɹnɚ tˈʊk ðɪ ɑːpɚtˈuːnᵻɾi ɐvˈeɪld tə hˌɪm æz kˈɑːɹdɪf pɹˈɪzən laɪbɹˈɛɹiən tə stˈʌdi ðɪ ˈɑːɹt ʌv wˈɔːɹ baɪ kˈɑːɹl vˈɑːn klˈɔːswɪts, ɐ wˈɜːk wˌɪtʃ wʊd sɪɡnˈɪfɪkəntli ˈɪnfluːəns hɪz fjˈuːtʃɚɹ ɐpɹˈoʊtʃ tə fˈɔːɹmjʊlˌeɪɾɪŋ jˈuːniən stɹˈæɾədʒi ænd klˈæs pˈɑːlətˌɪks ɪn dʒˈɛnɚɹəl ˈɔfən lˈiːdɪŋ tə fˈɜːðɚ kˈɑːnflɪkt wɪððə sˌiːpˌiːdʒˌiːbˈiː ɛɡzˈɛkjuːtˌɪv.
In 1932, imprisoned on trumped-up charges of unlawful assembly, Horner took the opportunity availed to him as Cardiff prison librarian to study The Art of War by Carl von Clausewitz, a work which would significantly influence his future approach to formulating Union strategy and class politics in general – often leading to further conflict with the CPGB Executive.