The United States Armed Forces, which at the time was literally the strongest most effective fighting force on the planet, was embarrassed by that tiny country.
The premise is that the clones are in the natural habitat of the ewocks, and therefore they're unfamiliar with their warstyle and unable to defeat them.
@MartinTheFirst Exactly. I'm not sure why this is such a hard concept for the fanboys to understand. Not to mention elite rebel strike team, Han, Chewie, Luke, Leia, etc.