Basically what happens is, you rescue Captain Price from prison after the Russians invade the US after the whole airport incident where the terrorist Markarov leads the undercover CIA American dead to take the blame for the whole thing. So, after rescuing Price from prison - you're in Washington DC fighting as one of the US Rangers. After clearing out the White House, you crash land in a Black Hawk and are fighting with your last clip of ammo. Just as you run out, a helicopter and a ton of Russians are rushing you, but it fades back to 'Roach' who is with Captain Price, Soap, and Ghost. You're invading this base where there is a submarine that has these 'nuclear missles'. Just before the sub is about to leave, Captain Price jumps in and takes it over. But as your defending the sub, a nuclear missle fires. Soap is yelling "WHAT IS GOING ON PRICE?!!! THE MISSLE FIRED!!!"
You hear Price saying "Good." as it fades out back to the US Ranger.

You replay the scene with the US Ranger in Washington DC with his last ammo clip, and suddenly a bright light appears. Suddenly, you're in outerspace as this astronaut who is looking at the earth. Then you see the missile as it closes in on the east coast of the US, and detonates. A satellite near you explodes and crashes into the astronaut, and you fade back into the US Ranger who is fighting.

Ends up, it was a massive EMP missile.