Waldoa and Dell19 are BOTH right.
Everyone knows that when a war happens between 2 civs and there is a huge gap in technology between them, the advanced one simply thrashes the backward. The only way the underdog can resist is guerilla warfare.
BUT after all this is a game, and playability is always more important than realism.
HOWEVER, blatantly ridiculous outcomes (like a trireme sinking a BB) should be prevented. And that would be easy to implement: for instance, each unit could have a "tech rank" associated to it, and when the difference between ranks would be too huge, the weaker unit COULD evade enemy fire, but not win the battle.
The bomber can always miss the spearman. The galley can evade the BB if the fight happens during a cloudy night and the BB has no radar. But there is no way the ancient units could kill the modern ones. I think implementing a possible "status quo" outcome would be a simple way to avoid absurd battle results and still give the underdog a (slim) chance to survive a few turns, until he gets to upgrade his troops.