"Defense" or "defence"

Advice / Advise, Licence / License are noun / verb pairs in BrE. The c or s endings denote whether the word is a noun or a verb. The reason in BrE why there is no word Defense or Offense is because the action of providing a defence is already taken care of by defend, same with offense" and offend. While one may advise by offering advice, or license a person by issuing a licence, one cannot defense a country, for example, by providing a defence - one defends a country. The 's' is still there in BrE with defence and offence in the forms - offensive and defensive so I would postulate that should there have been a need to have such a word defense in BrE, it would have been there. Side note: as an American it was excruciating writing all the ce endings and I have red squigglies all over my post!

At school in the U.K., in the 1970s, I was taught that defense is used in an active role and defence is used when in a passive role, just as "Scanner" says, above. (No ... defense is usually used, traditional British, when it is an active role: "his defense of the woman was admirable..."; whereas defence is used where it is passive "his only defence was...")

No ... defense is usually used, traditional British, when it is an active role: "his defense of the woman was admirable..."; whereas defence is used where it is passive "his only defence was..."

However the lexicographers tend to change things without general announcements :) so whereas people of my generation used both spellings (active/passive) to allow for the adjective "defensive", the OED at 2002 does not acknowledge "defense" as anything other than the US spelling.

When it comes to general usage though, anyone over 50 in UK will tend to use as I have described. So don't judge.

Maybe the dictionaries should have a newsletter? :D Once people have been taught such basic spelling and grammar in primary school they tend not to check the dictionary every time.

That is the only difference, yes. The British National Corpus and the Corpus of Historical American English have the following usage stats:

            BNC    COCA

defence   11709     570
defense     207   59677

Wiktionary marks defence as Commonwealth, and The American Heritage Dictionary marks it as Chiefly British.