Didn't say it was easy. Barring all Muslims for fear of Islamists, however, would put you on the side of such historically successful social policies as Jim Crow, the White Australia policy, and apartheid.The trouble is in distinguishing them. Jihadists seeking to enter a country don't red-flag themselves.
Let in 100,000 Muslims and even at 1% you have 1000 Muslims who might aid and abet Jihad or may actively carry out terrorism.
Hey, what's not to love?