Question: when did the vietnam war begin for the us
[{'answer': 'May 1 , 1950'}, {'provenance': [{'wikipedia_id': '8453309', 'title': 'Role of the United States in the Vietnam War', 'start_paragraph_id': 18, 'start_character': 0, 'end_paragraph_id': 18, 'end_character': 533, 'bleu_score': 0.9437815816129681, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5464197301572002841}, 'section': 'Section::::Timeline.:Harry S. Truman (1945–53).\n'}]}]