Or prevent you from defending yourself from an aggressive attack. If Russia was assaulted by US or NATO forces, the missile defense in Poland or anywhere near Russia could be used against their launches on any target; not just Poland.
Sure, but why would that ever happen? Seriously, the only reason why anyone would be attacking Russia is if Russia started some shit. If Russia doesn't plan to start some shit then they have nothing to worry about.
Still, I'm not convinced missile defense is feasible when you reach a certain level of technical complexity. Against obsolete and out of date missiles, there's more reliability, but against the most advanced nuclear missiles, it's very unsure.
Then again, Russia has little to worry about, since their missiles are much better than anything Iran or NK is likely to fire. Besides, I don't think there's too much in modern missile tech that makes them harder to shoot down, at least not the long range type. They mostly just go up and come down very fast.
But geographically it seems odd. If you put it in Poland, it seems much more likely that it's got something to do with Russia, with Poland being Russia's neighbor.
Unless maybe Poland is one of the eastern-most countries in Europe that we feel comfortable placing missiles in. It's not like Poland is the only place with missile defenses planned, we also have them in Japan and the west coast. The defenses in Poland aren't to defend America, they're to defend Europe, as part of our unpaid role as "world police" that everyone else just seems to take for granted. Besides, it sounds like the primary goal of that particular installation is less PRK, and more Iran, which would be more likely to attack Europe than the US. Keep in mind, we already have more limited anti-missile capabilities in the Baltic.
.and wouldn't aiming it over the Arctic region be a shorter trajectory?
I'm not positive, but I seem to remember something about it not working that way. Relative velocities and stuff. Like even if the crow-flies route is shorter, it's not faster or easier to fire a missile that way.