Where did this split between the two spellings come from? Well, it turns out that Americans used the "defence" version right up until about the early 20th century, when something caused them to change their minds:
This is a pretty justifiable switch, in truth; "defense" is probably derived from the Latin root "defensus," so it makes a bit more sense to spell it with the "S." Noah Webster, famous for the dictionary he made and the heartwarming 1980s sitcom it inspired, was a big advocate for spelling reform and probably advocated the "defence" -> "defense" switch on this basis.
But, justified or not, can you see how confusing this is as a hypothetical Canadian? Depending on the website or newspaper or book you read, there are two different, totally valid, totally reasonable and defensible spellings of a word. Heck, even if you're a well-read American -- I'm confident you exist -- you'll have seen both these spellings when you read foreign websites. And all because 100 years ago some asshole tried to fix a problem and other assholes didn't listen.
Let's take another example where you don't even have to pretend to be Canadian. let's look at, oh, let's say, the word "scissoring."
"Scissoring" doesn't have a "C" sound in it, does it? In fact, it probably used to be spelled "sissouring" or something similar. But some time around the 16th or 17th century, some dink decided to put the "C" back in to re-link it to its Latin root -- "scindere" -- which, whoops, turned out to not be "scissoring"'s Latin root at all. The spelling actually got harder because someone fucked with it!
So there you go, Internet. Not only do you now have a ready-built excuse when someone accuses you of spelling things wrong, but you've now got something fun to talk about while scissoring. You're welcome.
Chris Bucholz is a Cracked columnist and your best friend. Join him on Facebook or Twitter, where you might be able to elicit him to share his illicit collection of scissoring photographs.