How did mountains and rivers make it difficult for one group to control Europe?

Answer 1

Mountains and rivers form natural barriers that make it difficult for invading armies to cross. In addition, defenders positioned at the choke points of such barriers (mountain passes and either bridges or fords across rivers) have a huge advantage because only a few of the invaders can close with the defenders at a time. (That is the main reason the Spartans at the battle of Thermopylae were able to withstand a Persian army that was somewhere between 100 and 150 times their size for two whole days of battle. They held the only generally known narrow road thru the mountains so the Persians could only bring a tiny fraction of their army to bear at a time. If the Spartans hadn't be betrayed by someone showing the Persians another secret path around the Spartans, they might well have held out for who knows how many more days.)