You seem somewhat in denial as to the size and extent of the terrorist problem. There are 1.5 billion Muslims in the world, many of whom hope and pray for the fall of America and other Western countries. How many? No one really knows but its clearly a good percentage, whole countries in fact. 
Then there are the Muslims both here and abroad that are not terrorists themselves but may sympathize and silently condone the practice of attacking innocent people. They knowingly allow terrorists to hide among them and often provide material support as well.
This is no doubt a big problem for both America and Europe that will only continue to grow unless governments take steps to prevent more terrorists from entering and remove those already in the country.