Most countries in German have no article. A small group always takes one, and that changes the preposition too. Compare:

Ich komme aus England.

Ich komme aus der Türkei.

Which countries take an article?

Feminine: die Schweiz, die Türkei, die Ukraine, die Slowakei, die Mongolei. Country names ending in -ei are feminine.

Masculine: der Iran, der Irak, der Libanon, der Sudan, der Jemen.

Plural: Plurals make articles plausible for groups of islands or states: die Niederlande, die Philippinen, die USA.

Prepositions change with the article

This is where learners slip. Without an article, use nach for going and in for being: Ich fahre nach Deutschland. Ich wohne in Deutschland.

With an article, use in for both, with the accusative for going and the dative for being: Ich fahre in die Schweiz. Ich wohne in der Schweiz. Er fliegt in den Iran. Er lebt im Iran.

Why der Iran?

An explanation from the University of Marburg: Reza Shah introduced the name Iran internationally in 1935. French, which uses articles with countries (l’Iran), was then the main European language in Iran, and German took over the article. Usage remains mixed, and Duden lists the name with and without it.

Private German tuition in London and German lessons online cover articles like these from the first lesson.