Recently countries, where are emerging the increasing number of migrations, are Syria, Yemen where there is a war between the government and antigovernment. The reason their opposition is in complex of political and religious conflict. thus, extremism of Islamic criminal ring, named ISIS shortly, naturally expand their own influence in a tricky situation. They make a habit of doing illegal activities producing loss of lives such as terror. if the case that a terrorist disguise itself as a refugee and accept in anywhere countries, the terror will be openly happend in the country.
ISIS is the political and religious party in their country. it doesn't exist reasons or source to terror in any country.
in the case of Paris terror, it is the case of terror in other country. The reason of the terror was for showing off their desire and power. so, it is wrong that there is no reason to terror.