Why can defence be spelled 2 different ways?

You're right! "Defence" and "defense" are both correct spellings of the word.

This is because of a difference in spelling conventions between American and British English:

* American English: Uses the spelling "defense"

* British English: Uses the spelling "defence"

This difference applies to many words with "-ce" endings in American English and "-se" endings in British English.

So, both spellings are valid and correct, depending on the context and intended audience.