Back in 1860 as the United States was entering the Civil War, the southern states formed a Confederacy in which each state was in essence its own nation. States’ rights is what drove the South into secession and it was states’ rights that helped defeat the Confederacy. Why? Because each state, seeing itself as an independent nation, refused to centralize power in Richmond and instead focused on its own interests. Monies and troops were withheld, rail gauges changed throughout the South (making transnational troop transport difficult and inefficient), and each state had its own currency.
Which brings me to the Euro.
Europe began introducing the Euro back in 1999, in a sense forging one massive economic power consisting of multiple nations. We see in the 1860s, however, that the Southern Confederacy failed in large part because it wanted to be a confederacy of eleven separate nations. America tried this with the Articles of Confederation but moved to the current Constitution because a Confederacy was simply too weak for a nation stretching from coast to coast. If Europe is to remain a continent of separate nations, it must allow each nation to retain a national currency. The potential failure of the Euro will mean the survival of the European nations. The two cannot coexist.
UPDATE: The chief financial officer of the European Bank for Reconstruction and Development now says there are only seven working days left to prevent the collapse of the Euro. National sovereignty is the new wall standing in front of the Euro's survival, for European nations using the Euro are looking at a massive bailout by Germany - in which case the Germans will be able to influence the future economic decisions made the debtor nations. As it stands, the strong national identities of Europe may prevent the German bailout, thus bringing the Euro closer to collapse.
That is unless the United States offers its own bail out. Let's hope not.