There are experts on Hemingway on this site but so far none have answered and this is a question which is more general. You are right about the meaning and that "so" would make the meaning clearer. The absence of "so" in this expression is old-fashioned and literary, and Hemingway is probably borrowing an expression from the Bible and/or Christian hymns relating to the sacrifice of Jesus Christ. You can see this if you search online.

I agree with Michael. The additional literary analysis is that the protagonist writes in irony. He mocks himself, the Bible and the war. He is not particularly patriotic. He is not in any mortal danger. His own experience of the war has not been particularly dangerous. His contributions are not so important to Italy. Italy is not surviving particularly well. The entire sentence is ironic and somewhat frivolous, and is meant to be humorous.  The biblical references are: "But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God." (Matthew 4:4) Hemingway's "I live by sight drafts" mimics and mocks this verse and is intended to be ironic and humorous. "Who (Jesus) died for us, that, whether we wake or sleep, we should live together with him." (1 Thessalonians 5:10) You see the omission of "so" here. The deeper philosophical question is: "Are patriotism, war and religion essentially and ultimately meaningless?" What's the point? To the characters in the novel, the the most important and real aspects of life are the relationships and the daily round of food and drink and conversation. This question is particularly pertinent in these times that we live in.

Gordon, and anyone who is remotely interested in Hemingway, should read this insightful article on his life and suicide. http://www.independent.co.uk/news/people/profiles/being-ernest-john-walsh-unravels-the-mystery-behind-hemingways-suicide-2294619.html

(Ben moved this comment to "Answers" below.)

Thank you, Bill.