Suppose the Taliban manages to secure stable control over a sizable swathe of Afghanistan’s Pashto belt. Then suppose a terrorist detonates a bomb in a western city and kills hundreds of people. Where do we think that terrorist is more likely to have come from: Kandahar or Rotterdam? My money’s on Rotterdam. Or Paris or London or Hamburg. In part this is just common sense; a radicalized Dutch Muslim is already in the West making it a relatively simple logistical matter to detonate a bomb in Amsterdam. And it’s even fairly simple to fly from Western Europe to New York or Washington—you don’t need a visa, etc. Afghanistan is on the other side of the planet.
Which isn’t to say we should invade Europe instead. But merely that we need to keep the balance of risks in perspective.