Once again, the British left is at war with itself. The cause of this most recent conflict: the war in Ukraine. The territorial dispute to be settled: who are the real bad guys here? Putin’s Russia, or the US and its Nato allies? Or is it perhaps both? Friendly fire is flying in all directions, injuring the reputations of all who come into contact with it.

In an interview with the Double Down News website last month, Jeremy Corbyn defended the controversial campaign group Stop the War, which is no fan of Nato. The former Labour leader’s comments served to “defend a bunch of genocide deniers and Putin proxies”, wrote the activist and journalist Paul Mason, previously a Corbyn supporter. “You sound like an unhinged McCarthyite,” responded a fellow leftist, the Guardian columnist Owen Jones, at the end of a lengthy Twitter exchange.

This is not a new battle. Mason, a former Trotskyite, says he has been fighting within the Labour Party for years. “Internally, we fought and decisively won a battle to keep Labour pro-Trident and pro-Nato,” he wrote on Twitter.

Inevitably, lurking on the fringes, are the conspiracy theorists. The comedian and actor Russell Brand is perhaps the left’s most high-profile proponent of pro-Russia conspiracies. “You’ve Been LIED To About Why Ukraine War Began”, screams one video that has garnered 2.7 million views on his YouTube channel.

James Ball, a former WikiLeaks and Guardian journalist who co-hosts the podcast The New Conspiracist, notes that such thinking stems from the far left’s scepticism of mainstream media. “If you don’t believe anything the western media says, you often end up inevitably pushed towards taking a pro-Russia line,” he says.

Lunatic fringe aside, where does this instinct come from, which opposes Nato and, its critics say, gives succour to Putin? “For older socialists, there is often a sense that they are fighting the last war,” says Ball. In their eyes, Russia still means the Soviet Union, and the urge towards a viable alternative to capitalism moves them towards support for the only alternative that has been tried.

For younger people on the left, who grew up in the shadow of the Iraq War, the case is simpler. In their eyes, the US is the world’s imperial power and, since imperialism is bad, US-backed Nato must be bad too. Seen through this lens, the fact that eastern European countries wish to join Nato is seen not as a voluntary embrace of western values but as an expansion of the US empire.

David Lammy, Labour’s shadow foreign secretary, highlighted this false dichotomy in a speech to an American audience. “For too long, parts of the left, even some members of our own party, falsely divided the world into two camps: America and the West on one side, and their victims on the other. This has never been right, but this view has now been exposed for all to see as a farce.”

This is fighting talk from Lammy, turning his sights on his own side. Yet his boss, Sir Keir Starmer, has made it clear his team is up for waging this battle to the bitter end. When a group of MPs on the party’s left, including the former shadow home secretary Diane Abbott and the former shadow chancellor John McDonnell, signed a letter from Stop the War criticising Nato, Starmer threatened to withdraw the whip from them. The MPs duly removed their names, their brief stand crumbling against cold reality. Corbyn, already freed from the Labour whip, kept his name on the letter.

“Let me be clear,” Starmer told a meeting of the parliamentary Labour Party (PLP) at the end of February, “there will be no place in this party for false equivalence between the actions of Russia and the actions of Nato.” Since his intervention, the PLP seems to have fallen into line. “The Labour Party has changed profoundly in the last two years. Our support for Nato is unshakeable,” he told Radio 4’s World at One.

When it comes to Ukraine, Starmer is taking a position largely indistinguishable from that of the Conservatives. He welcomed the prime minister’s package of sanctions against Russia, but called on the government to go harder and faster. On refugees, he called its efforts “too slow, too narrow, too mean”. Do what you are doing, seems to be his message to the Tories, but do more of it. Allies of Starmer say he is following his natural instincts. His position will also probably prove to be an electoral asset.

On his own side, however, he is engaged in a game of political Whac- A-Mole. The National Education Union (NEU) gave the party leadership a further headache last week after delegates rejected a motion calling for a “negotiated settlement in Ukraine” and voted against adding a clause to the motion that declared that the people of Ukraine “have a right to defend themselves against this invasion”. While the NEU is not officially affiliated with Labour, its position does give a sense that there are still those on the left who are not toeing the party line.

In his quest to change his party, Starmer has won some significant early victories. But every time it looks as if peace might break out, a new skirmish begins. The conflict is in its early days, though, and as every good general knows, what matters is not who wins the battle, but who wins the war.