Greece has a smaller GNP than Indiana. How can it cause all this trouble? Just let it go!
The same way that a small hole below the water line in a ship can sink the ship.
If you let the water pour in long enough there will come a time when the ship can no longer be saved.
Unfortunately, the economy of Greece is not the only hole in the ship. Like the passengers on the Titanic, the rest of the world is watching Greece, on one end of the ship, wondering if the same fate awaits the rest of the world at the other end of the ship.
But the european (and US) banks own tens of billions in Greek debt. If the Greeks default, the banks may fail. Which will lead to European Hank Paulsons predicting riots in the streets unless the European governments give them blank checks.