Let’s dispel another spelling mystery. It’s defense against defence, and if you think it’s one of the British English vs. American English things—you might be onto something.
Defence and defense are both correct ways to spell the same word. The difference between them, the fact that one’s spelled with a “c” and the other with an “s”, comes down to the part of the world in which they are used. In the United States, people spell it with an “s”—defense. An American would write something like this:
In parts of the world where British English is used, they use the spelling with a “c”—defence. A Brit would write:
This difference in spelling carries over to the inflected forms of the word only partially. In words like “defenceless,” “defencelessly,” or “defenceman,” the British spelling retains its “c,” instead of changing it for an American “s”—”defenseless,” “defenselessly,” or “defenseman.” But when the suffix added to the word begins with an “i,” in both American and in British English the resulting word is spelled with an “s”:
It should also be noted that you might come across the word defense being used as a verb. This is particularly common in the US when talking about sports, when the word is used instead “defend against”:
This usage is considered incorrect by some style guides, and it would be better if you didn’t use defense as a verb outside the context of sports.
from Grammarly Blog https://www.grammarly.com/blog/defence-defense/