If he was a war veteran, he would lie.
If he were a war veteran, he would lie.
with the former indicating a higher likelihood that he really was a war veteran?
'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.
I've noticed recently that Wilkie Collins in The Woman in White (1860) was not keen on the subjunctive form either:
"He flatters my vanity by talking to me as seriously and sensibly as if I was a man."
"… he addressed me oratorically, as if I was laid up in the House of Commons."
"He exerted himself to interest and amuse us, as if he was determined to efface from our memories"
But he also writes:
"... as if he were still a young man"
"I turned and looked after him, wondering if he were ill or out of spirits."
According to Merriam-Webster's Dictionary of English Usage, many writers use both was and were in counterfactual conditionals, sometimes both in the same sentence.
That's why the subjunctive is virtually dead, because native speakers don't feel it makes a difference. Originally, as is still the case in languages where the subjunctive is a big deal, the "if he were" construction would show a much higher degree of reticence, doubt, fear, or reluctance to evoke the clause in question (cf. "God forbid she should get sick now, at the age of 6 months," vs. Let's try and avoid letting her getting sick at the age of six months.)
Actually, I think it's just because we're bringing the verb be in line with every other verb. With every other verb, we use the past tense in present counterfactuals: if I owned a car, I would go for a drive. It's only be that has this special form were, and it's only distinguished from the regular past tense in the first and third persons. So it's not surprising that it's being replaced by was.
This doesn't mean that we don't see the need in expressing counterfactuals. It just means that we don't use a special verb form to do it. I'm not sure what doubt, fear or reluctance has to do with it.
"If he was a soldier" can means two different things - i) "If he used to be a soldier in the past" and ii) If he was a soldier now.
Do you mean that this is impossible in your dialect?:
A: "Do you think he would lie?"
B: "He used to be a soldier."
A: "Ah, if he was a soldier, he would lie."
"If he was part of the gang that robbed that store, he must be arrested."
"If he was at the party [last night], he would know the truth."
How do you express past factual conditionals?
A: Ah, if he used to be a soldier, he might lie.
"If he was at the party last night, he must know the truth."
The presence of "would" in the main clause leans the whole sentence towards being an unreal conditional in my mind. Or maybe I've lost all perspective because I'm thinking about this too much.