If the environment where people live is destroyed or degraded then the people who live there will move elsewhere. The most important factor in destruction or degradation of the environment is humanity. By greed humanity destroys the environment in particular places; it cuts down forests, it creates vast quantities of carbon dioxide as a by-product of the things it does to satisfy its greed and it makes war for reasons of fear and greed.
Greed will degrade the environment slowly, sometimes so slowly that those who inhabit it do not notice the degradation until it has arisen. Those living in a slowly degrading environment caused by greed will remain overly optimistic of the future and try to stay in familiar surroundings as long as those and they can sustain some life. They often think that things will improve, and things may improve.
But war degrades the environment very quickly and fills those who inhabit war zones with pessimism and caution, thus they rush to move out of the war environment. They do not want to risk their lives and the lives of their children to the vagaries and chances of war.
This is how the migration crisis that Europe is presently experiencing has arisen and that is how it works. The migration crisis is not just about people fleeing war, but those fleeing war seem to make up the largest numbers of those who migrate. The great Western democracies must take a large measure of blame for the state of affairs in Syria, Iraq and Afghanistan; like old shopkeepers’ rubric “if you broke it, you bought it” and so we did.