The decision by voters will worsen Greece's economic turmoil, and though its leaders argue that a "no" vote is simply a mandate for further negotiations, it is widely believed that the vote will accelerate the country's progress toward an exit from the eurozone.
But while the economic impact of the crisis is already being felt across Europe, the security implications of a "Grexit" are at least as ominous as the financial ones.
First, it is almost certain that a Greek exit from the eurozone will push the country closer to Russia, deepening divisions within NATO. Greek ties with Russia have been improving since the victory of the left-wing Syriza party in January's elections, and Greek leaders have made no secret of the fact that they consider Moscow a possible alternative source of funding if their European negotiations fail.
Syriza's traditionally strong relations with Russia have been strengthened through a series of recent visits between Moscow and Athens. During his visit last month at the St. Petersburg International Economic Forum, for example, Greek Prime Minister Alexis Tsipras spoke of the Greek and Russian relationship
, hinting that Greece was "ready to go to new seas to reach new safe ports."
Russian officials deny that they have offered financial aid to Greece, yet the Russian energy minister just recently announced a $2.77 billion pipeline project in Greece
, and Moscow followed this with an informal invitation to Greece
to join the BRICs' New Development Bank.
It is true that the initial impact of a Grexit would simply be to provide Russia with a wonderful propaganda coup, allowing anchors on Russian state TV to highlight further evidence of the decline of the European Union and of Western civilization more broadly. Yet a closer Greece-Russia relationship would also have bigger long-term security implications for the European Union and for NATO.
Though the European Union's Ukraine-related sanctions on Russia were renewed without objection on June 22, that extension is only good through January. Greek leaders have strongly objected to sanctions on Russia in the past. There is no precedent for a member state leaving the Euro, and Greece may remain a member of the European Union even if it leaves the single currency..Greek leaders would have little to lose in choosing to veto any future extension of the sanctions on Russia.
Even if Greece were to exit the European Union entirely, it would remain a member of NATO. Greece was a Cold War battleground and an important player in the struggle against communism. Yet today, a Greek government with strong ties to Russia would not only perpetuate the Russian narrative that NATO is weak and divided but could easily exert veto power over NATO operations or any NATO response to potential Russian aggression in the Balkans or Eastern Europe.
Greece also has a strategically important location in the southeast of Europe and on the Aegean Sea. Closer ties between Greece and Russia raise the possibility that Athens might permit Russian ships the friendly use of Greek ports. This would be a major strategic concern for NATO, allowing Russia to expand its military influence not only in Crimea and the Black Sea, but to obtain a stronger foothold in the Mediterranean. The Greek-friendly Cypriot government, though not itself a NATO member, already permits such use.
It's even possible that a Grexit and closer Athens-Moscow ties could lead Greece to withdraw entirely from NATO, moving instead toward closer security cooperation with Russia.
There is precedent: Greece has previously withdrawn from NATO, from 1974 to 1980. While relatively unlikely, a new Greek withdrawal from NATO would result in the loss of strategically placed bases, placing an increasing burden on Turkey to support NATO's southern security needs.
To be sure, despite implied promises, it is unlikely that Russia could entirely bail out Greece in the event of their exit from the euro. After all, Russia is suffering substantial economic problems as a result of low oil prices combined with American and European sanctions and might itself face balance of payment problems in the next few years.
But given Russian President Vladimir Putin's demonstrated willingness to accept domestic economic pain to further foreign policy objectives, the coming months will likely see favorable financing deals and Russian support for major infrastructure projects -- such as the recently agreed Turkish Stream pipeline -- as well as increased cultural and political ties between Athens and Moscow.
Eurozone leaders will meet in an emergency summit on Tuesday to determine Greece's future.
Sunday's referendum results will leave them with a dilemma: They can offer a more generous, politically unpopular bailout deal in an attempt to keep Greece in the eurozone, or begin the process of a costly and damaging separation.
In doing so, European leaders and their U.S. counterparts must bear in mind not only the financial fallout, but also the unsettling security implications of an EU and NATO member state slipping into Moscow's orbit.