Suppose that BitCoins are being widely used all across the globe. Suppose that all internet connections between two countries are blocked (eg China and US go to war) and people still engage in transactions inside each network. Now all transactions within each network are broadcasted to all nodes inside its network, but not to the other network. Within each network, the longest chain in each would be considered valid, and the BitCoin economy would continue to exist inside each network.
Now after several years existing independently, what happens when the two networks are reconnected?