Learn German: Du musst vorsichtig nach Frankreich fahren.

The preposition nach is always followed by the dative case, and it is the standard word for "to" when traveling to most countries, cities, or continents. However, because most countries like Frankreich are neuter and don't use a "the" word (like der, die, or das) in front of them, there is no article to change into the dative case! You just say nach Frankreich.

Why is the verb fahren pushed all the way to the end?

This happens because of the modal verb musst (must). In German, the conjugated modal verb takes the normal second position in the sentence, and it pushes the main action verb (fahren) to the very end in its dictionary form (the infinitive).

Why doesn't vorsichtig have an ending, like vorsichtige?

In this sentence, vorsichtig is describing the action of driving, which makes it an adverb (meaning "carefully"). In German, adjectives and adverbs look exactly the same, and they never take an ending when they describe a verb. They only get endings when they come directly before a noun.

Can I use fahren if I am flying or walking to France?

No, fahren specifically means traveling by a wheeled vehicle or boat (like driving a car or taking a train). If you were flying to France, you would use fliegen, and if you were walking anywhere, you would use gehen.