It was the year of raised hopes, of surging idealism, and of brutal disappointments and cold realities. I began the year among dancing crowds in Athens, jubilant at the great Syriza-led revolt against EU-imposed austerity. I am ending it in Madrid, where Syriza’s ally Podemos hopes for sweeping gains. It feels as though I’ve come full circle.

The West’s political landscape is increasingly polarised. On the one hand, there is the quasi-fascist demagoguery of Donald Trump, the right-wing populism of Ukip and the xenophobic nationalism of France’s Front National and the Scandi far right; on the other, the American socialism of Bernie Sanders, the progressive civic nationalism of the SNP, the anti-austerity insurgencies of Syriza and Podemos and, of course, Lab­our’s grass-roots-led Corbynite turn.

In western Europe, the explanation seems relatively straightforward. Social democracy, in its established form, is imploding. Its original social base – an alliance of the industrial working class and the progressive middle class – has fragmented. Social democrats’ acceptance of pro-market nostrums in the 1990s and 2000s left their political mission ever vaguer. When mainstream social democracy – however reluctantly – embraced austerity, attacking its own supporters and leaving it without any clear purpose (what is a social democrat who doesn’t support public investment?), it left a desperately large vacuum. Like nature, politics abhors a vacuum, so this has been filled.

But for those of us with hopes for a new left that is politically viable, 2015 has been an instructive lesson. Passion isn’t enough, events told us; Greece told us that even power wasn’t enough. Syriza inspired people like me not just because Greece had been left shattered and immiserated, its youth robbed of jobs and its hospitals of medical supplies. We thought that if Syriza won concessions from its tormentors, that would embolden similar political forces across Europe.

Unfortunately, the EU’s leaders may be ruthless but they are not stupid, and these hopes were their fears. Under no circumstances could the revolt of tiny Greece be seen to succeed in any way, or it would spread. Donald Tusk, the president of the European Council, admitted it was “political contagion” rather than “financial contagion” that was feared from Greece, and he warned of 1968-style unrest sweeping the continent. A courageous and overwhelming rejection of the EU’s austerity package by the Greek people – OXI! – in a referendum led to short-lived euphoria. The most radical left-wing government ever democratically elected in Europe was compelled to implement the most right-wing economic policies on the continent. Syriza had led a revolt; now it was leading a semi-colonial regime, implementing policies it violently rejected, at the behest of foreign powers.

The Greek tragedy proved what should already have been incontrovertible: that it is impossible to transform one society in isolation. Thanks to the globalisation of capital and a European Union institutionally wedded to the dogma of the market, this has never been more true. The EU has to change if the ambitions of the left are to be realised. That requires forging stronger links between progressive movements across borders. Language and cultural barriers don’t make it easy, but in the age of mass travel and electronic communication, such solidarity is far more possible and tangible than ever.

And then there’s Britain. The Miliband era was a striking chapter in the crisis of European social democracy. Five years of ideological mush that satisfied nobody; the failure to offer any coherent, let alone compelling, vision; the failure to rebut the myth that Labour’s spending threw Britain into economic crisis: all these ensured that Britain was returned into the Tory fold.

In other Western societies, there are often two mass-left parties: one social democratic and one of the radical left. (In Spain, there is the social-democratic PSOE and the radical Podemos.) It’s an exaggeration to claim that if the Parliamentary Labour Party is PSOE, the Labour membership is now Podemos, yet there’s a great deal of truth in that. Tens of thousands flooded into the party’s membership and people queued around the block to hear Jeremy Corbyn speak in the summer. The same discontent that has been sweeping the Western world latched on to Corbyn’s unlikely candidature in England and Wales, inspiring a chunk of the Great Disillusioned. A man with worse betting odds to become leader than yours truly ended up with the most impeachable democratic mandate of any party leader in history.

That was the easy bit. Labour now had a leadership with little experience in either mainstream politics or media, colliding with a hysterically aggressive press and a Tory party dripping with McCarthyite invective about threats to national security. Unforced errors – inevitable in such a context – followed.

And here, too, the left has to learn. When the newly elected Labour MP Jess Phillips recently told me that good communication doesn’t mean being a Blairite, she was right. It is possible to have ideas that can transform the country – rather than accepting the prevailing consensus – and also be media-savvy. Tory smears about Corbyn being a threat to national security might seem laughable to tweeters, but they can resonate in the wider world. Hence the need to emphasise that the left’s intent to rid Britain of injustice is an inherently patriotic act.

So many raised hopes: so many frustrated dreams. Yet the lesson of 2015 is not to despair, but to learn. The left suddenly has a mass audience again and it can succeed. But unless it learns from its setbacks and defeats, then it will fail. And it is the people it exists to represent who ultimately suffer the consequences.

Owen Jones is an NS contributing writer