The Ewok Effect is the name of a phenomenon in which an highly trained and merciless army with equiped with superior weapons and terrifying combat vehicles are beaten by a bunch of poorly organised stone age primitives.

The name of the phenomenon is (obviously) derived from the scenes in the Return of the Jedi movie in which the evil Imperial Stormtroopers (armed with blasters, Speeder Bikes, AT-STs and AT-ATs) were defeated by the Ewoks (armed with spears and rocks).