Use the full diplomatic name

Warning²Federal courts, Illinois courts21st and 22nd editions

An ICSID award names each state party as the state styles itself; a bare country abbreviation is shorter than any diplomatic style, whatever style the state uses.¹

1. 1R. 21.6 International Arbitrations and Claims Commissions. Tables: T10.3 Countries and Regions.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.