GENEVA — More than 1 million people driven out of their countries by war, poverty and persecution entered Europe in this record-breaking year, migration experts said Tuesday, a symbolic milestone capping a mass movement of people that has challenged the concept of European unity. The migration organization’s director-general, William Lacy Swing, urged European governments to make migration safer. Most people entered Europe via Greece, which took in 820,000 people this year, nearly all of them crossing from Turkey by boat across the Aegean Sea.