The Siege is for lack of better words a war on Asgard by Norman Osborn and his Initiative. They have led an assault the likes of which have not been seen before. They took down Thor, almost to the point of death in the last issue. In the last panel you see a reflection in Iron Patriots armor and realize it was Caps shield as the panel draws closer.
Cap shows up with his Avengers to try and save the day. Now with the "war" at a more playing field, Norman Osborn feels the reigns of power slipping. With Osborn facing defeat he cries out "to cap's Avengers telling them they don't know what they have unleashed. The Sentry transforms into some kind of monster.
I don’t really know what’s going on with them with The Sentry. I’ve liked him since his first limited series and this is the only problem I have with the books so far. They are ruining The Sentry. Otherwise it’s been a great limited series and has gotten me back into Marvel Comics.