D
Deleted member 231381
Unconfirmed Member
The biggest flash point will be over territorial waters, it's flared up a couple of times when both were members of the single markets and common fisheries policy, it flared up regularly pre 1986 and pre 1992 and will become a big issue when the UK leave especially if Spain isolates Gibraltar.
The UK's claim on territorial waters are bit weaker than their claim on the port itself, and basically boils down to we've been here a while now. The Treaty of Utrecht only gave Britain the forts and port, it didn't give them the isthmus to the north.
At time periods this long, self-determination is generally the main legal determinant in international decisions and courts. If every territory was arbitrated on the basis of legal documents from the 18th century, we'd be fighting the Fourth, Fifth, and Sixth World Wars before we hit 2020. Spain don't really have much of a leg to stand on.