"Travel To" vs. "Travel In" — What's the Difference?

The preposition changes the meaning:

• "Travel to [country]" → You are going from another country to that destination.
  Example: "I will travel to Japan next month." (You're currently outside Japan.)
• "Travel in [country]" → You are moving around inside that country.
  Example: "We traveled in Japan for two weeks." (You were already there, touring.)

Quick Test

Both are grammatically correct; they describe different phases of a journey.