Complex sentence: As a young man, known for his honnesty and vulnerability, he left his parents home to serve in the U.S. army during WWI, he was told that his health was so bad that he would not survive another American winter.
Split into simpler sentences: As a young man, known for his honesty and vulnerability, he left his parental home to serve in the U.S. army during WWI. Doctors told him that his health was so bad that he would not survive another American winter.