Keeping Greece in the euro: Mission impossible?

Greek Prime Minister Alexis Tsipras meets German Chancellor Angela Merkel in Berlin later on Monday.

Tsipras arrives in Germany -- the biggest single contributor to Greece's 240-billion euro ($262 billion) international bailout -- warning that Athens will find it "impossible" to avoid defaulting on its debt repayments without more cash from Europe.

"Servicing these repayments through internal resources alone would, indeed, lead to a sharp deterioration in the already depressed Greek social economy -- a prospect that I will not countenance," Tsipras wrote in a March 15 letter to Merkel first published by the Financial Times.

Greek government spokesman Gavriel Sakelarides confirmed the content of the letter Monday. He said Tsipras was not threatening Germany but rather laying out the "reality" facing his country.

"There is a liquidity problem and political initiatives must be undertaken," Sakelarides told Greek TV.

And here's the problem: Europe will only hand over more cash once it's satisfied that Greece is implementing reforms it promised in February to win an extension of its financial lifeline to the end of June.

Pressure on Merkel to hold a firm line with Greece is coming from two sources: German taxpayers who don't want to throw good money after bad, and European states who have stuck with painful austerity and believe they're beginning to see the benefits of those reforms.

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While the political wrangling continues, Greece's financial position goes from bad to worse. Tax revenues fell short of forecasts by more than one billion euros in the first two months of the year, the economy is shrinking again and cash is leaving the country.

"Tonight, Prime Minister Tsipras has potentially his last chance to convince German Chancellor Merkel that he will ultimately do what it takes to keep Greece in the euro," noted Christian Schulz at Berenberg bank.

"If he fails to inspire any kind of trust in Berlin, securing the necessary funds to keep going, let alone the inevitable third bailout in July, will be a fantasy."

Greece repaid more than 900 million euros to the International Monetary Fund last week, but faces another repayment demand of about 470 million euros on April 9.

With its budget surplus fast evaporating, and unable to borrow from international markets, Greece's anti-austerity government would then face a stark choice: default or slash spending at home.