Question: when did the vietnam war start for the us
[{'answer': '1950', 'provenance': [{'wikipedia_id': '32611', 'title': 'Vietnam War', 'start_paragraph_id': 2, 'start_character': 504, 'end_paragraph_id': 2, 'end_character': 508, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]