knowledge - What is a Union Word™?

_{This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.}

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After a long period of division, tends to unite; After a long period of union, tends to divide. This has been so since antiquity. - The Three Kingdoms

If a word conforms to a special rule, I call it an Union Word™.
Use the following examples below to find the rule.

$$ % set Title text. (spaces around the text ARE important; do not remove.) % increase Pad value only if your entries are longer than the title bar. % \def\Pad{\P{0.0}} \def\Title{\textbf{ Union }} % \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\def\V#1{\S{#1}{9}} \def\T{\Title\textbf{Words}^{\;\!™}\Pad}\def\NT{\Pad\textbf{Not}\T\ }\displaystyle \smash{\lower{29px}\bbox[yellow]{\phantom{\rlap{rubio.2019.05.15}\S{6px}{0} \begin{array}{cc}\Pad\T&\NT\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline % \text{ ARGUMENT }&\text{ PARAMETER }\\ \hline \text{ DEUCE }&\text{ DRAW }\\ \hline \text{ PANASONIC }&\text{ HITACHI }\\ \hline \text{ CANESUGAR }&\text{ HONEY }\\ \hline \text{ FRAGMENT }&\text{ SCRAP }\\ \hline \text{ NORTH }&\text{ SOUTH }\\ \hline \text{ BRANCH }&\text{ DIVISION }\\ \hline \text{ INDEX }&\text{ EXPONENT }\\ \hline \text{ BELLY }&\text{ BACK }\\ \hline \text{ SAUCEPAN }&\text{ WOK }\\ \hline \end{array}$$

And, if you want to analyze, here is a CSV version:

Union Words™,Not Union Words™
ARGUMENT,PARAMETER
DEUCE,DRAW
PANASONIC,HITACHI
CANESUGAR,HONEY
FRAGMENT,SCRAP
NORTH,SOUTH
BRANCH,DIVISION
INDEX,EXPONENT

BELLY,BACK
SAUCEPAN,WOK

Hint 1:

Nation

Answer

A Union Word is a

Word whose first three letters are a valid Alpha-3 country code.

ARGUMENT

ARG -> Argentina

DEUCE

DEU -> Germany

PANASONIC

PAN -> Panama

CANESUGAR

CAN -> Canada

FRAGMENT

FRA -> France

NORTH

NOR -> Norway

BRANCH

BRA -> Brazil

INDEX

IND -> India

BELLY

BEL -> Belgium

SAUCEPAN

SAU -> Saudi Arabia