If Charles is a person's name, the proper form would be Charles's defense
just like it would be Mary's defense. The apostrophe only form (e.g. the girls' coat) is for plurals.
So, Wales' new flag and Wales's new flag
are both acceptable.
There are lots of quirks with apostrophes that you should study.
For example, mothers-in-law's is the correct possessive form of the plural
noun mothers-in-law. Mother-in-law's is the correct singular possessive
form. (Both end 's. )