Question: the united states' war on terror began in the wake of which of the following events
[{'answer': 'September 11 attacks', 'provenance': [{'wikipedia_id': '13425800', 'title': 'War on Terror', 'start_paragraph_id': 1, 'start_character': 160, 'end_paragraph_id': 1, 'end_character': 180, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '13425800', 'title': 'War on Terror', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1091, 'bleu_score': 0.901900932797118, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18120434838830948865}, 'section': 'Section::::Abstract.'}]}]