Said elsewhere: along with the correct spelling of "realise"

Now I know Richard is teasing here. We all understand that each version is legitimate and proper, this being one of the standard differences-across-the-pond.

But what I'm asking here is, how did this difference arise? I seem to recall that it was some hypercorrection of what was assumed to be a French word - or something like that?

Interestingly, only realize, not realise is in the OED. I wonder why.

The OED prefers the "ize" ending and uses it as its house style. If you read the introductory notes to the dictionary they explain their reasoning which is basically that, for most formations, "ize" and "ise" are both acceptable in UK English and that OED prefer "ize" as it is the earlier formation. I suspect the "ise" is now the more popular but, as OED suggest, consistency is more important than spelling preference.

There is one main exception to the rule of preference: the "yse" ending (analyse) is always used in UK English whereas the "yze" ending is used in US English. This is not a matter of choice; it's a genuine difference.

Actually -ise is the earlier formation, from Old French -iser.

Richard, do you say that the "ise" is more popular because Canada, the UK, Australia, and New Zealand use it?

I think 'ise' is the more popular throughout the non-US part of the English-speaking world, although a sizeable minority use 'ize', including the Oxford University Press, who have adopted it as their house style, as Richard said. As we know, 'ize' is the only version used in the US.

Is this the transatlantic position?

To Americans -ize or –ise
Should not occasion surprise;
A Brit’s –ize may sneak out
When he’s seeking to tout
Favour from you American guys.