On this day, 44 years ago, tanks sat outside La Moneda, the Presidential palace in the Chilean capital of Santiago. After three years of social struggle and US-backed economic sabotage, the Chilean armed forces, led by General Pinochet, extinguished the democratically-elected Marxist government of Salvador Allende. The president died clutching a gun given to him some years previously by Fidel Castro. With him died the "peaceful road to socialism" – the Chilean left’s strategy for bringing about radical socialist change through the electoral process.

If, as seems increasingly likely, Jeremy Corbyn becomes Prime Minister, it is difficult to imagine similar scenes outside 10 Downing Street. But the anniversary of the Chilean coup should be a cause for reflection for the left. The Corbyn project might appear new and unique, but in government it will face the same massive forces that other left governments have faced – from Allende in Chile to Syriza in Greece. So far, the British left has done almost nothing to prepare for this fact.

From the moment Jeremy Corbyn gets into office, his government will face a campaign of sabotage. Labour’s 2017 manifesto was not a call for socialist revolution, but it does represent a final radical break from the neo-liberal economic consensus and significant shift in wealth and power towards ordinary working people. The establishment is right to fear Corbyn, and it will resist him. Capital will threaten to flee the country (as banks threatened after post-crash regulations were introduced), civil servants will refuse to implement policy, and a tag team of establishment bigwigs and hostile media outlets will declare his every move dangerous and illegal.

The real aim of this pressure will not, at least immediately, be to overthrow the government. It will be to extract compromises, and surrender if possible. Britain is not Greece, but Corbyn will come under huge pressure to give up on core policies, and to implement orthodox economics and austerity. Many of Syriza’s leaders were also highly principled socialists, drawn from the most impressive grassroots left in Europe. They buckled under the seismic pressures of government when they signed yet another bailout deal in the summer of 2015.

And for Corbyn, the temptation to compromise on key policies will be exacerbated by a parliamentary Labour party still overwhelmingly populated by people who, at least initially, opposed them in the first place. Tony Blair is not the only prominent Labour figure to genuinely believe Corbyn’s programme to be ruinous. Many MPs will fall in line, but many others have operated as part of the same political establishment that Corbyn hopes to overthrow. When crunch votes come, will Corbyn really be able to rely on his parliamentary colleagues, and how far will he compromise to retain their loyalty?

On both fronts – sabotage and compromise – the dynamics of seeking power and exercising it successfully run almost directly counter to one another. To win and then maintain power, Salvador Allende had to profess total faith in Chile’s democratic and military institutions. To unite and give hope to the Greek people in 2015, Alexis Tsipras had to pretend that he might be able to keep his promises and remain in the Eurozone; to really deliver on his promises, he would have needed to prepare the population for Grexit.

Corbyn’s political programme is relatively moderate, and unlikely to cause the same level of desperation among the ruling class that Allende caused in Chile. He can also rely on a less vulnerable economy than that of Greece. But winning the election will still be the easy part. Unity in the parliamentary party might appear key for winning elections, but to really have a parliamentary majority, Corbyn will need many more left-wing MPs than we currently have.

The sight of thousands cheering Corbyn on at Glastonbury this year provided a powerful reminder of the sudden politicisation of hundreds of thousands of people in Britain. Unless harnessed effectively, however, this freshness will turn into a weakness. The most effective way to build immediate support is to simplify and cheerlead. What is really needed is a whole generation of independent, thinking activists who understand the pressures that the Labour leadership will face in government. The Glastonbury crowds must, in time, be prepared for a gruelling fight with the establishment; they must be prepared to hold the leadership to account and push it from the left.

Jeremy Corbyn is not yet the Prime Minister, and winning power will remain the priority. But the experience of the left in government, from Chile to Greece, teaches us that we cannot be naïve. Under the surface of the campaign to win power, the grassroots base of the Corbyn movement must prepare itself for the realities of what will face it in government.