The US doesn't need to annex Greenland, just set up a few bases there which can be more easily done. But even that has less benefits now than it had during the Cold War. If one wants to intercept hypersonic ICBMs they need satellites equipped with powerful lasers to zap'em. Kintetic interceptors in Greenland or space won't do.
Dunno, it has to be some energy beam because kinetic ammo needs replenishing. In space. So laser, microwaves, EMP etc. Maybe it could ignite the ICBMs fuel somehow, as most of the missile's payload is the fuel.
