Question: when did us involvement in the vietnam war start
[{'answer': 'spring of 1950', 'provenance': [{'wikipedia_id': '8453309', 'title': 'Role of the United States in the Vietnam War', 'start_paragraph_id': 1, 'start_character': 561, 'end_paragraph_id': 1, 'end_character': 575, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '8453309', 'title': 'Role of the United States in the Vietnam War', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1423, 'bleu_score': 0.8342572995323291, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18188613817108326957}, 'section': 'Section::::Abstract.'}]}]