As a key security partner for the West and the prime route for migrants arriving in Europe, Greece poses a threat to more than the financial stability of the continent if its crisis deepens.
All eyes this week are on whether Athens will finally reach an agreement with its creditors and save its financial system from collapse.
But experts warn a potential Greek exit from the eurozone -- "Grexit" -- poses broader security threats.
The first, immediate concern is the ever-increasing flood of migrants trying to get into Europe.
Greece recently overtook Italy as the leading point of entry with more than 48,000 illegal border crossings there from January to May, according to the EU's Frontex agency.
"If Greece is substantially weakened, it will no longer be able to manage its responsibilities in an effective way," said Thanos Dokos, head of the non-profit Hellenic Foundation for European and Foreign Policy in Athens.
"There is already a shortage of fuel for coastguards, and no prospect of modernising equipment.
"We can imagine how these things will worsen after a Grexit, with much less funds for controlling borders or intercepting suspicious persons, which is already difficult."
- Russian influence? -
Greece's future matters to the West in a more general sense thanks to its location on Europe's southeastern flank -- a position that has made it a vital asset since it joined NATO in the 1950s.
The US has carte blanche at the Souda military base on the island of Crete, using it for intelligence-gathering and as a launch-pad for operations throughout North Africa and the Mediterranean.
It used a string of other Greek bases for air strikes against Libya in 2011.
That contrasts with neighbouring Turkey, which has increasingly put restrictions on the US air force base in Incirlik in the south of the country.
"Greece is the only country in the region that has been in the EU since the 80s and in NATO since the 50s," former Greek army chief Christos Manolas told the EU Observer online newspaper recently.
"It is a security provider rather than a security consumer."
Although NATO is unlikely to lose the use of bases any time soon, the prospect of Greece falling out of the eurozone, tumbling into even deeper economic crisis and growing ever-more alienated from its Western allies has jangled nerves on both sides of the Atlantic.
Some fear the chaos in Greece could be exploited by Russia as it tries to divide European countries.
"(Russian President Vladimir) Putin could help Greece with humanitarian aid in exchange for coming under Russia's influence," said Andrea Montanino, director of the global business and economic programme at the Atlantic Council think tank.
"That becomes a problem because decisions in Europe on things like sanctions often require unanimity."
The distractions could not have come at a more inopportune moment, with the West facing a multitude of crises from the war in Ukraine to the jihadist threat of the Islamic State group.
"Europe has been devoting a lot of its energy to the Greek crisis, leaving less room to focus on very important challenges like Ukraine, which has been growing more unstable on a daily basis," said Heather Conley of the Center for Strategic and International Studies in Washington.
- Criticism of US -
Conley has been sharply critical of the White House for failing to see the wider implications of the Greek crisis and its threat to US security interests.
"The Obama administration has diagnosed this as an economic crisis, when in fact it is political.
"The administration continues to be surprised by the north-south divide opening up in Europe, the extremism, the anti-global and anti-US feelings that have emerged," said Conley.
"We need strong partners, and Europe is undergoing a profound transformation. The greatest military alliance in history doesn't just run on automatic, and we have been leaving the Treasury to deal with this crisis."
If Greece leaves the eurozone, it could plunge the country into a full-blown humanitarian crisis, triggering social unrest and boosting the chances of its own ultra-nationalist party, Golden Dawn.
Golden Dawn "is becoming cleverer, gradually replacing its Neanderthal-like faces with more ordinary even respectable ones, much like Marine Le Pen in France," said Dokos.
"That makes it even more dangerous."