Using en with feminine countries and au(x) with masculine countries to say in or to (prepositions)

We know that countries and continents have genders in French [see Continents, countries, regions & states are masculine, feminine or plural (gender)]

Note that while we always use in and to in English for cities and countries, in French we use different prepositions for one or the other.
We use à with cities Using 'à' (to/in) and 'de' (from/of) with cities (prepositions), it's a bit more complicated when it comes to countries.

Have a look at these examples:

Here are the rules:

ATTENTION: 

For pronunciation reasons, you will use en with masculine countries starting with a vowel:

Note that Le Québec behaves like a country, even though it's a province:

See also the related lesson: En, dans = In, to with regions, states, counties (prepositions)