Jehovah`s Witnesses like to pick and choose what scripture they will obey and those they don`t like or contradicts their beliefs. So you will have an uphill battle with this question.
First off J.W`s do believe in war ,they believe in the" War of the Great day of God Almighty at Armageddon" by Christ Jesus/Jehovah God.
That will kill billions of people man woman and child simply because they are not Jehovah`s witnesses anytime soon.
Jehovah God is well known as being a war God in the Old Testament. He sanctioned many wars including genocide of many peoples and Nations.
In the New Testament Cornelius an army officer of the italian band was favoured by Jehovah God and accepted while he was in the army and there is no indication he ever left or was required to do so.
I wish you well in talking to your father ,but i`m afraid it will fall on deaf ears .
There is a scripture in the Bible that is applied to false gods & Idols ,however I find it aptly applies to Jehovah`s Witnesses.
Psalm 115: 5-8
"Eyes they have but they cannot see,
Ears they have but they cannot hear,
and a mouth they have but they cannot speak."
that is The Truth About The Truth , ( TTATT) but that`s just the way I see things.
And please let us know how you get on.