What primarily set off large-scale migration flows among European countries during the twentieth century?

The economic crisis in most of the European countries was one of the primary reasons for migration in European countries.

Explanation:

Earlier after the end of second world war, many people migrated from country side to developed countries in search for work, with the intend to get a better life. Many unskilled labors got job in developed countries after this world war.

But when countries started facing economic crisis many workers started migrating to their home countries. There was land shortage and job shortage at this time. Farmers faced crop failure

Another cause of this migration was civil war that took place in Spain.

Most people migrated to seek religious, political and personal freedom.

This migration was also impacted by global conflicts also.