You can’t always use “en”. When you need to use “à” + country, you’ll need to use “au” if the country’s name is singular masculine and begins with a consonant sound, “en” when it is singular feminine or singular masculine and begin with a vowel sound, and “aux” when it is plural.

Le Danemark, l’Afghanistan, la Belgique, les États-Unis

become

Au Danemark, en Afghanistan, en Belgique, aux États-Unis.