I am trying to create a validation rule that will make the Account Billing Country and the Contact Mailing Country fields required and contain values from a specific list (i.e. Country = Argentina or Bolivia or Colombia and nothing else). So far I have only been able to make the fields required. I am having trouble defining the correct formula for the pick-list of acceptable values. I tried:

AND( LEN( BillingCountry ) = 0, CONTAINS("Argentina || Peru || Bolivia || USA || Canada || Ghana || Indonesia || Malaysia || Taiwan || Mexico || Nigeria || Pakistan || South Africa || Uganda || India || Philippines || Armenia || Russia", BillingCountry))

But it is not giving me an error message when I mis-spell one of those countries nor when I enter something not on the list.

Any help would be appreciated.

Thanks