[ekianjo]

Napoleon wanted to do a naval blockade against England. It was possible to do it through the countries he now controlled, but Russia made it impractical to implement such a blockade since they were too big to be influenced in other ways.

Russia was not sitting still, While Napoleon was busy fighting in Spain, Russia with the Tsar Alexander was attacking Sweden and Turkey - they were just as expansionist as ever. There were rumors that Russia was going to march on Warsaw next, too. Russia was also preparing for a larger army to go further into Europe. Both countries were preparing for a clash, and it does not really matter who stroke first.

The same thing happened between Hitler and Stalin in 1941: they distrusted each other's and both eventually had plans to turn against each other.