No one factor explains the difference, but I’d compare the danger along several dimensions. The first to consider is simply the goals of the Islamic State, though this is ultimately unsatisfying. As I’ve argued elsewhere, in contrast to Al Qaeda’s anti-U.S. emphasis, Islamic State leaders have primarily focused on their state-building project in Iraq and Syria. Developing provinces in the Muslim world is another goal. Although most Islamic State attacks still strike regional targets, attacks on the West have risen on the priority list, particularly after the United States and allies in Europe began an air campaign against the Islamic State. But given that the United States is the leader of the coalition (and supplying the vast majority of the strike assets), Washington should be at least a rung above Europe on the enemies list. And within Europe, Belgium should be relatively low on the ladder given its minimal contribution to the anti-Islamic State campaign, even discontinuing military operations late last year. So looking solely at the Islamic State’s enemies list is not enough.
It’s more useful, then, to focus on how easily the Islamic State can strike Europe in comparison to the United States. And here the contrast is clear. Over 5,000 Europeans have gone to fight in Iraq and Syria; less than 200 Americans have joined the struggle. Europe also shares a land border with Iraq and Syria via Turkey and is logistically far more accessible for Islamic State fighters to go back and forth, while the United States is protected by two great oceans. Even putting foreign fighters and simple geography aside, Muslim communities in Europe have more radicalized individuals who stay in Europe, and thus a greater native pool of recruits for lone wolf and other attacks.
Beyond foreign fighters and radicalized stay-at-homes, the integration challenge is far greater in Europe. The specifics vary by country, but almost everywhere, the situation is bad. Contrary to conventional wisdom, there is no direct correlation between education or poverty and terrorism. However, when such problems are rampant within one community, they suggest a problem that is apparent to anyone who walks a Muslim neighborhood in a major European city: a lack of integration...
