Question: when did the usa join the korean war
[{'answer': 'June 1950', 'provenance': [{'wikipedia_id': '16772', 'title': 'Korean War', 'start_paragraph_id': 1, 'start_character': 98, 'end_paragraph_id': 1, 'end_character': 107, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '16772', 'title': 'Korean War', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 1028, 'bleu_score': 0.513567763769745, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3829313818443109774}, 'section': 'Section::::Abstract.'}]}]