Despite the momentous events in Greece over the weekend and a continued slide in Chinese stock prices into bear market territory, the reaction from the U.S. on Monday has been more muted that the European markets' response. 

Imagine not being able to withdraw more than $67 per day from your bank account, regardless of the sum it contains. Or waiting in long lines at petrol stations, supermarkets, and general appliance stores. That is reality for ordinary Greek citizens. Already suffering the painful effects of a protracted depression, the Greek economy is now on the verge of a debilitating financial crisis. 

Consider that, on the basis of the decline in real gross domestic product, Greece is now worse off than the United States was in 1933. And it now seems probable that the Greek economy will deteriorate further, at least in the short term, regardless of whether it remains in the eurozone. 

The permanent crisis in Greece finally took a decisive turn over the past three days, with the country's future as a eurozone member in real jeopardy. Prime Minister Alexis Tsipras ended talks with Greece's creditors on Friday, then on Saturday announced a July 5 national referendum on their final bailout proposal (he is calling on Greeks to reject it). 

On Sunday night, Tsipras was back on TV to warn that banks would be closed (at least) through July 6; combined with capital controls, this is meant to prevent a collapse of the banking system. The latter is now comatose, kept alive only through emergency funding from the European Central Bank -- which the ECB capped (but did not withdraw -- a crucial distinction) over the weekend.

This is what a financial crisis looks like. Following months of negotiations and repeated extensions of multiple bailout programs spanning years, things are now moving very fast -- the next three weeks will prove decisive to Greece's economic and political future, as well as that of the euro currency union and, indeed, the entire European project. 

Greece's current bailout program expires on Tuesday, at which point the nation will certainly fail to repay €1.6 billion owed to the International Monetary Fund (this will not technically constitute a default). On July 20, Greece must redeem a €3.5 billion bond held by the ECB; nonpayment here would be a default. 

Although the Greek stock market was closed on Monday, you can still get some sense of what might have been: The Global X FTSE Greece 20 ETF (NYSEMKT:GREK), which represents the top 20 companies by market capitalization on the Athens Exchange, was down 15.6% at 12:15 p.m. EDT. For reference, the Dow Jones Industrial Average's largest one-day decline was 22.6% in the crash of 1987, followed by a 12.8% plunge in 1929.   

However, U.S. investors can remain calm: Greece is an economic minnow, less than a half-percent of the global economy; there is little reason to believe its crisis will have any significant impact on the U.S. Still, a Greek exit from the eurozone would set an unnerving precedent, proving the union is not "irrevocable" and highlighting some of the contradictions in its architecture.  

It appears that Tsipras has badly misjudged his negotiating "partners" and misplayed his hand. The next few days and weeks ought to prove absolutely fascinating, but this is a European problem -- I think it will take more than this to shake the U.S. market from its torpor. While the degree of complexity and interconnections in global markets makes that difficult to predict, even if the volatility in stock prices were to pick up, genuine (i.e., business-focused) investors can be confident that U.S. business values will remain stable as a rock through this crisis.

Alex Dumortier, CFA has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.