In January 2015, Alexis Tspiras stormed to power as a firebrand of the radical left. He vowed to wage war against the Greek oligarchy, stand up to the EU technocracy and strike fear into the hearts of investors around the world.

“Greece leaves behind catastrophic austerity, it leaves behind fear and authoritarianism, it leaves behind five years of humiliation and anguish,” he proclaimed to a throng of supporters on election day in 2015.

But that was then. In the four years that followed, Tsipras tried desperately to endear himself to the establishment he once pledged to fight. He protected the old oligarchs and ushered in a generation of new ones. He implemented austerity measures so brutal that even Germany’s finance minister Wolfgang Schäuble accused him of “putting the burden on the weak”. And he placated international investors with big promises of small taxes and golden visas.

“Reforms are like a bicycle,” Tsipras told the Financial Times. “If you don’t [make] them, you fall down.”

There is a common view that the Tsipras transformation was predestined. Among rightwing observers, it is portrayed as the natural byproduct of radical politics colliding with cold reality: Tsipras simply got wise to the adolescence of Syriza’s confrontational stance. Among leftwing observers, it is portrayed as the inevitable result of the EU’s anti-democratic architecture: Tsipras had no choice but to abide by the troika of lenders’ diktat – Syriza’s dreams were dead on arrival.

Syriza betrayed its principles – and the Greek people. Its days are numbered | Alexander Kazamias Read more

But this view radically underestimates both Tsipras’s agency as prime minister and the extent to which he willingly flung himself to the far end of the political spectrum.

True, the circumstances of his premiership were not easy: the troika was determined to make an example out of Greece. And the constraints of global capitalism were more difficult still: strike too hard at the ship-owning oligarchs, Tsipras was warned, and they will simply leave the country. We can sympathise with many of the difficult decisions that come with high office.

But nothing was predestined about the eviction of struggling families and the foreclosure of their homes. Nothing was predestined about the auction of vast tracts of land and sea to fossil fuel corporations such as ExxonMobil. Nothing was predestined about the severe overcrowding, sexual violence, and shortages of “doctors, medicine, food and drinking water” in Greece’s migrant camps. And nothing was predestined about the sale of arms to Mohammed bin Salman, the smiles of support for Benjamin Netanyahu and the purchase of fighter jets from Donald Trump.

In short, Tspiras did not simply capitulate to the troika, or swap his radical ideals for hard-nosed realism. He actively refashioned his government as a rightwing force on the world stage.

For many Syriza supporters, the betrayal was too much to bear. Scores of Greece’s young people, Syriza’s historic base, swung to the centre-right New Democracy instead: from just 16.5% in 2015, New Democracy’s share of the youth vote rose to more than 30% – an ironic outcome, given Syriza’s commitment to extending the franchise to 17-year-olds. The party’s performance among pensioners was even worse: according to exit polls, 46.4% of retired Greeks voted New Democracy, compared to just 25.5% for Syriza.

Play Video 0:46 Greece is not Europe’s ‘poor relative’, says Greek PM-elect after landslide victory – video

In May, Tsipras made a last-ditch effort to woo these voters back to the party with a “stimulus package” of tax cuts and pension bonuses – essentially adopting New Democracy’s economic programme. But this move only heightened the dissonance between Syriza’s populist rhetoric and its parliamentary reality. When election time finally rolled around, voting for New Democracy at least allowed voters to resolve this dissonance – to sense a return to reality, however harsh, from Tsipras’s circus celebration of a nonexistent economic recovery.

What lessons now can we learn from Syriza’s demise?

I propose three – about policy, party and politics.

The first is not to pander to the opposition. A common explanation for the decline of leftwing parties over the past quarter century is their inability to speak to rightwing concerns. But Syriza shows that mimicking the right does little to slow its rise. On the contrary, Tsipras’s flirtation with militarism and neoliberalism caused his party to lose its core identity, emboldening its opponents along the way. Why settle for Syriza’s centre-right pandering, many voters asked, when you could have the real thing?

The second is not to invest the hopes of a movement in the figure of a politician. It is a common refrain in Greece that “Tsipras is Syriza”: he is the only one with the political wiles to hold together the disparate factions of the Greek left. But like a pyramid flipped on its head, the personification of Syriza’s platform made its politics highly unstable, liable to tip in any direction that Tsipras chose to lean. Only a party that is sufficiently grounded in a social movement – where the base of the pyramid holds its head to account – can remain true to its founding principles, particularly in a context as volatile as Greece.

And the third is to keep the flame of rebellion alive. Syriza rose to power on its promise to challenge the status quo courageously and creatively. “Hope is coming,” its slogan assured. Voters understood that the chances for radical change were slim – but they trusted Tsipras to fight anyway. When Syriza became the purveyor of injustice instead, that spirit of rebellion was lost, and its base of support collapsed along with it.

• David Adler is a writer and a member of DiEM25’s Coordinating Collective. He lives in Athens, Greece