Synonyms for "overwhelming force"
Solution 1:
The first thing that came to mind to fill the blank in your example sentence was:
“ … with all its might.”
However, that phrase would probably be overstating the actual/literal extent of the attack and even so overstated, it doesn’t necessarily imply, as “overwhelming” does, that the power of the attacking force disproportionately exceeded that of the force being attacked.
Granted, your use of "empire” and “rebel [base]” in the example sentence would probably lead one to assume that “all the might” of an Empire would be more than enough to defeat/overwhelm “all the might” of a rebel base. As you say yourself, however, your example is not only just an example, but a trivial one at that.
All that to try to say that without the “clues” of “empire” and “rebel base,” attacking “with all its might” does not guarantee that the defender’s might is not equal to or even greater than that of the attacker.
I do, however, think that replacing “force” with the noun sense of “might” would be a good first step in finding a suitable synonym for “overwhelming force” and that combining "might" with the complete/thorough/extreme sense of “crushing” could overcome the problems with “all its might”:
The empire attacked the rebel base with [its] crushing might.
Might (from oxforddictionaries.com)
NOUN
[mass noun] Great and impressive power or strength, especially of a nation, large organization, or natural force:
‘a convincing display of military might’
Crushing (from * macmillandictionary.com*)
ADJECTIVE
1 complete and achieved very easily
a crushing defeat/victory
Complete and thorough: thorough, total, pure...2 very severe
a crushing blow:
It’s a crushing blow for the president’s foreign policy.
Serious, severe and extreme: the nuclear option, serious, severe...
Here are two example uses of “crushing might” that seem to me to be synonymous, or nearly so, with “overwhelming force”:
“The coordinated assaults against Germany which had been fixed upon at Yalta went forward with crushing might.”
(from Churchill-Roosevelt-Stalin: The War They Waged and the Peace They Sought By Herbert Fei, via GoogleBooks)
“[We are rendered] helpless [and] paralyzed [with] the crushing might of the state … hurled against any man who refuses to accept the new Faith.”
(from Fictions of Power in English Literature: 1900-1950 By Lee Horsley (quoting Czeslaw Milosz from The Captive Mind) via Google Books)