[quote=@Gentlemanvaultboy]World: Super Dimension Fortress Macross/Robotech.
What Changed?: Someone sabotages the three Zentradi spies ship, stranding them on the Macross. They never make it back and expose their comrades to Lynn Minmay. Without that, there is no dissent among the Zentradi ranks and the human race is going to get pasted when they start fighting for real.
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Note that humanity is going to go down HARD here. Like, multiple Alaska Cannons. It was the cultural infection of the Zentraedi that motivated bringing in the main fleet that toasted Earth in the show, and if that takes longer, then Earth as more time to prepare and build up defenses. Maybe not enough to win, but they're going to be able to do quite a bit of damage.