Defending your nation vs attacking another, the defender has an advantage. Is that unfair? I wouldn't say so.
Whatever advantage one side's got, its unfair for the other. Its severity can, however, be disregarded as decisive or 'overpowered'.
When the Arabs invaded us, we had traps and height advantage and it was, without a doubt, unfair for all their thousand dead-ones - in opposition to our few ones.
When Japan ignored the treat and attacked the US it was unfair because the US army was sleeping and lost lives without being able to defend themselves. It was unfair, even though they pushed back and nailed the japanese.
A handycap is always unfair, that's why you cant start a war silently anymore. That's why ONU is banning nuclear bombs. Its not because they cant deal with them, its because its unfair to just have them. Many other examples could follow...
Dont forget, in your example, you stated no variables. One country can have an advantage while the opposing country can have other and they can, somewhat, cancel out each other or make it fair, in a way.
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fair != just