Is the only difference that in USA they write it with s and in UK they write it with c, or is there anything more?
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Wiktionary marks defence as Commonwealth, and The American Heritage Dictionary marks it as Chiefly British.
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.