In some cases, the definite article is omitted before the name of a country or region. The exact rules can be difficult to figure out, even for natives.
In expressions relating to the government of a country or region, the definite article is usually omitted, if the name starts with a vowel or is feminine.
Le Royaume de Thaïlande. (La Thaïlande)
Le Royaume de Belgique. (La Belgique)
La République de Chine. (La Chine)
La République d'Allemagne. (L'Allemagne, féminin)
Le Royaume d'Afghanistan. (L'Afghanistan, masculin)
With a masculine noun starting with a consonant, the single-word form du remains.
L'Empire du Japon. (Le Japon)
Le Royaume du Congo. La République du Congo. (Le Congo)
Le Royaume du Danemark. (Le Danemark)
Until some time around the classic age (until the 18th century? I don't know exactly when the cut-off was), masculine forms also used de rather than du, but depending on the country this either has disappeared or is disappearing. While some prescriptivists hold on to le Royaume de Danemark and le royaume de Portugal (and I wonder why these two specifically), they are no more warranted than le royaume de Congo (decidedly old-fashioned) or *l'Empire de Japon (didn't take on until it was too late for de).
For some non-monarchic regimes, the adjective is used instead of the noun: la République française, la République espagnole, la République italienne, la Confédération helvétique, … But la République d'Allemagne, la République d'Irlande Maybe the republic has to have existed since the 19th century? I'm not sure about this.
This also applies to heads of state who personify the country, as opposed to heads of state who represent the people.
Le roi de France
Le roi de Belgique
Le shah d'Iran
L'empereur de Russie
L'empereur du Japon
Le sultan du Maroc
but le président français, irlandais, chinois, congolais, …, or “le président de la RDC [République Démocratique du Congo], le président de la Corée du Nord” when the country needs to be named.
There is a nuance between “le roi de France” and “le roi des Français” (lit. King of France vs. King of the French). The roi de France claims his authority from his own might or from God. The roi des Français claims his authority from a mandate of the people. To say that people revolted over this is only a slight exaggeration — while it wasn't the cause of the revolutions per se, it was a very important symbol.