'Was' is indicative past tense (relating to a fact about the past); 'Were' is subjunctive past tense (relating to a hypothetical about the present).
Unfortunately (or not), once a war veteran, always a war veteran. If you replaced this with "soldier", there would be a difference in meaning.
If he was a soldier (during the war, but not necessarily now - past indicative), he would lie.
If he were a soldier (now - subjunctive), he would lie.
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