Start by naming any country that France borders. Then name any country that borders any of those countries. Then keep going until there are no more countries left to name.
Not sure if my end, but didn't manage to do this with ease. Assumed it's the capitals highlighted by the blue dot, but the blue dot disappeared too often, so couldn't work out what was required. Love the concept though!
It'd be nice if you fixed it so that Vatican isn't directly under Rome. It's hard to scan the screen really quickly to see what capital is active already, and having to worry about it being hidden really hinders the speed.