https://www.orwell.ru/library/essays/Spanish_War/english/esw_1
The full part:
In the present war we are in the curious situation that our ‘atrocity campaign’ was done largely before the war started, and done mostly by the Left, the people who normally pride themselves on their incredulity. In the same period the Right, the atrocity-mongers of 1914-18, were gazing at Nazi Germany and flatly refusing to see any evil in it. Then as soon as war broke out it was the pro-Nazis of yesterday who were repeating horror stories, while the anti-Nazis suddenly found themselves doubting whether the Gestapo really existed. Nor was this solely the result of the Russo-German Pact. It was partly because before the war the Left had wrongly believed that Britain and Germany would never fight and were therefore able to be anti-German and anti-British simultaneously; partly also because official war-propaganda, with its disgusting hypocrisy and self-righteousness, always tends to make thinking people sympathize with the enemy. Part of the price we paid for the systematic lying of 1914-17 was the exaggerated pro-German reaction which followed. During the years 1918-33 you were hooted at in left-wing circles if you suggested that Germany bore even a fraction of responsibility for the war. In all the denunciations of Versailles I listened to during those years I don't think I ever once heard the question, ‘What would have happened if Germany had won?’ even mentioned, let alone discussed. So also with atrocities. The truth, it is felt, becomes untruth when your enemy utters it. Recently I noticed that the very people who swallowed any and every horror story about the Japanese in Nanking in 1937 refused to believe exactly the same stories about Hong Kong in 1942. There was even a tendency to feel that the Nanking atrocities had become, as it were, retrospectively untrue because the British Government now drew attention to them.
This is an astonishing assertion, one unbacked by citations. Is this true? I can't imagine it's true, and yet I can't imagine Orwell would lie. Are these attitudes something that went down Orwell's "memory hole"?
When WWI happened, the Socialist Parties across the world splintered into pro-war camps and anti-war camps. The moderate Socialists who supported national defense or national wartime solidarity merged with right-wing or moderate socialist groups, while anti-war Socialists moved farther to the left.
In Orwell's case, he's referring to the far left British Socialists that emerged after the war. Many of the right/moderate socialists merged with the Labor Party.
He's pointing out that the government's anti-German propaganda pushed the far-left into a pro-German mentality, regardless of factual analysis. They were hostile to anything the Government propagated, even if it was true.
He's not referring to the overall/broad left. He's referring specifically to the British Socialists who were essentially Bolsheviks after the war.
Rather than dig for hints of seemingly pro-German sentiment in the anti-WWI Left, it would be more worthwhile to understand the actual Left critique of the war itself, which is not hard to find. In the wake of the Bolshevik Revolution in Russia, V.I. Lenin was held very high regard. The people Orwell is criticizing would almost certainly agree wholeheartedly with Lenin's analysis of WWI:
https://alphahistory.com/russianrevolution/lenin-on-world-war-i-1914/
It's also important to really read closely what Orwell is saying here. He's not saying the Left-wing line on WWI was literally “pro-German”. His point is that for anti-war British Leftists, their default attitude was so knew-jerk anti-nationalist, that they veered into being *virtually* pro-German because they so absolutely rejected the mainstream pro-war discourse, to a fault.
One last crucial bit of context: Orwell's harshness toward the Left was because he himself was a Leftist. He ruthlessly mocked the rigid dogmatism of Communists, but we should never forget that he had recently volunteered (at age 33) to fight for a Marxist party against Franco's forces in Spain. He was horrifically wounded and very nearly died. The hypocrisy of fellow Leftists bothered him so much because he believed in their cause.