Defense with an 's' was in use in British English for centuries before the Pilgrim Fathers even set sail for America; indeed, it was used alongside the 'c' version. Eventually, however, they settled down in such a way that we stuck with the 'c' and the Yanks stuck with the 's' and it remains thus.
I'm so glad you pointed this out, Quizmonster. I used to rail against what I regarded as 'American' spellings until I read Jane Austen, in whose work people constantly 'realize' things and 'categorize' others. I think 'color' was originally ours, too (not quite sure). But certainly settlers took the spelling of the day to the New World. It was we who subsequently changed it and then started laughing at them!