I just rewatched the first Star Wars, sadly mangled by Lucas after the fact. Episode IV, as it's now known.
The big bad threat was the Death Star. We know it's bad because it destroyed Alderaan. Boom!
And the climactic end of the film had our heroes attacking it before it could get in range of the moon their base was on and destroy it. There was a planet in the way, you see.
Soooo, seeing as we already know it can blow up planets ... why didn't they blow up the planet that was between them and the moon at the very beginning? And then blow up the moon? Seems like a pretty good strategy to me, wouldn't you agree?