(CNN)Europe was hit in 2015 with a historic wave of humanity -- people, many from the Middle East and North Africa, fleeing violence or poverty, seeking a place to live, a job and a chance in life for their children.
4 reasons the refugee crisis will shake the world in 2016
European Union procedures groaned to the breaking point. With the EU's external borders as porous as a sieve, exercising zero control over internal borders -- those between two EU members -- no longer seemed such a grand idea.
The free movement of people throughout the EU came under threat.
On the migration issue, European unity seemed an unreachable goal. Germany welcomed the newcomers. But Hungary built a wall.
And the issue will only increase in importance in 2016, for several reasons:
The biggest group of people seeking safety in Europe came from Syria, where one of the modern era's most brutal civil wars is grinding toward the five-year mark with no end in sight.
By some estimates, a quarter of a million people have been killed. According to the United Nations High Commissioner for Refugees, nearly 4.4 million people have fled the country.
Talks are taking place. But when on one side you have President Bashar al-Assad, accused of torturing and gassing his own people, and on the other the appalling terrorist group ISIS, there seems little hope of a negotiated solution.
The war will likely continue. And the stream of refugees looks set to grow still larger.
Bernie Sanders would rather talk about closing the gap between rich and poor. Rick Santorum might prefer to talk about banning abortion.
But the tide of migration sweeping over Europe, combined with the terrorist attacks in Paris, has ensured that migration will play a significant role in the 2016 U.S. presidential campaign.
Republican presidential candidate Donald Trump has called for building a wall along the border with Mexico and for banning Muslims from entering the country. And he's riding high in the polls.
Whatever their issue of preference, the candidates in 2016 will have to talk about migration. And they may well find it to be a minefield.
The idea that foreigners are coming to the UK to take advantage of the taxpayer-funded National Health Service, and to take jobs away from native Brits, has long played a role in British politics.
The refugee crisis has only increased those concerns and bolstered the anti-immigration, anti-EU UK Independence Party, or UKIP.
British Prime Minister David Cameron has promised to hold an in-out referendum -- a nationwide vote on whether Britain should stay in the European Union or get the heck out -- make what's known as a "Brexit." He has said he will hold it by the end of 2017, but many observers think he will do it this year.
If so, fears over increased immigration would likely increase the chances that Britons would vote to leave the EU and try to go it alone in their island fortress.
For a group called the European Union, the EU was remarkably disunited in 2015 when it came to the migration crisis.
It could not come up with a united response. A plan to resettle refugees by setting quotas for each EU country to accept went nowhere.
Beyond that, the idea that there may be jihadis hidden among the migrants entering the EU gained currency with reports that one or more of the Paris attackers had sneaked into Europe posing as a refugee.
The signature achievement of the European Union, which began as a free trade zone, has been the free movement of people and goods. You can cross the border between two EU countries (with some exceptions) in your family sedan or your semi-trailer without a border check.
But if the outer borders of the EU cannot keep out dangerous people, then countries within the EU will take to patrolling their own borders again. The free movement of goods and people will have collapsed.
And with it, the essence of the European Union.