The tension between Europeans and immigrants has been percolating for the last year. Stories that aren't mentioned in the U.S. news outlets include mention of ski-mask covered vigilantes going after Middle Eastern immigrants in shopping malls. They do this for retaliation for the groping of the European women that happens on a daily basis. The violence has already started.
Tensions between Eastern European and Middle Eastern immigrants in Western Europe might rise to the level of violence. Especially after the New Years incident in Germany there is a high possibility. Western Europeans are themselves facing a threat in their own countries and women and children now have to be particularly careful. Several women who were attacked on New Year's eve have been scarred for life. Migration does bring with it a number of consequences including social instability and hence a breakdown of law and order.
While people in Eastern European countries seemed to be welcoming at first, the economic reality is starting to hit them. These are people who are poor anyway, and they will not stand for long for social services money to go to someone else. When people get desperate, they often resort to violence.
This is a tough one - and more like trying to predict the future than answering a debate question. But I'd like to hope that each set of immigrants starts being grateful for what the countries they have fled to have begun providing them, rather than taking out their frustrations on their fellow immigrants.