If Greece leaves the euro, the U.S. economy could easily slip into recession. That cost is worth bearing, however, given the long term consequences of Europe struggling with a currency regime that makes little sense. 

Essentially, Athens would exchange drachmas for euros in circulation among its inhabitants, remark private debts and bank accounts to drachma, and until conditions stabilized, limit withdrawals at Greek banks and capital outflows.

As the drachma fell in value, Greek exports would increase, reducing unemployment.  consumers and businesses, with their debt redenominated into a cheaper currency, would enjoy a windfall and spend more. All would help lift the Greek economy out of crisis, but this can only happen if Germany and other European governments cooperate.

The money Greece owes other governments, the European Central Bank and International Monetary Fund-under international law-can only be remarked to drachma with their consent, but that debt is too substantial for Greece to repay in euro.