Am I the only one that thinks that a large number of levels and concepts for these games were taken straight from a number of films?? I mean don't get me wrong, I dont think I could come up with a completely engrossing and compelling story line from scratch but to many action fans like myself felt like that some of the story and in particular the levels had been lifted from things like Mission Impossible 3 and the Rock in a major way.
Shepherd lost men in the first one as it was mentioned before, around 30,000 or something, so he is pretty mad and set out on revenge, this blank cheque will allow him invasion of the enemy soil responsible for the death of his men. Price has launched the nuclear warhead which exploded in the upper atmosphere, it send out the electro magnetic pulse, which disabled all electricity on the Eastern Coast of US, which allowed the US soldiers to repel the Russian forces, the Russians had pretty much overwhelemed the Americans with the helicopters etc etc, outnumbering them technology wise, this pulse had disabled all type of technology, so doing a favour to the Americans. However the space mission shows that the International Space Station was destroyed as a result of the EMP.
I think Shepherd just generally got the blank cheque for the invasion of the Russians, seeing as in the game US weren't prepared to begin with.