Which of the following nations was NOT a belligerent in World War I?

The story behind the answer

Denmark is correct because it remained officially neutral throughout World War I. In contrast, Greece, Romania, and Portugal all entered the war on the Allied side: Greece after the National Schism (1917), Romania in 1916 aiming to claim Transylvania, and Portugal in 1916 after maritime clashes with Germany, later fighting on the Western Front (e.g., Battle of the Lys).

Denmark, bordering Germany, chose strict neutrality to avoid invasion and safeguard trade. It mobilized defensively, managed the crucial Danish Straits, and balanced commerce with both blocs while enduring blockade pressures. Memory tip: the three southern European countries fought; the lone Scandinavian country—Denmark—stayed out.