Seeing some other alien race run into Captain Fantastic, when he's backed up by his war veteran sidekick and his power is doubled. That's when you play the united defense card and see the Jet fighters pile in as well. Those Menthalar saucers never had a chance!
The down side is when this happens near the bottom of the deck and you know Captain fantastic is coming for you next. Good thing we came in peace!
The last twice I tried that particular manoeuvre, on a city or the Capitol the darned aliens then played "We Come In Peace!" and won the battle with out a fight! and no one had a cancel card!!!