Adolf Hitler only decided to join the Nazi party after being snubbed by another far-right party, a prominent historian has discovered.

Thomas Weber, a historian based at Aberdeen University, said if Hitler had not been rejected from the newly established German Socialist party, it is unlikely there would have been a world war or Nazi Germany’s mass murder of six million Jews.

Dr Weber, who has been researching the Nazi leader for more than ten years, unearthed an unpublished document which reveals that the German Socialist party told Hitler in 1919 that they did not want him in their party or to write for their paper.

The academic, who is a Professor of History and International Affairs, argued it is not likely Hitler would have risen to power if he had been allowed to join the German Socialist party. He said it would have been harder to climb party ranks in what was a larger and more successful organisation than the Nazi party where Hitler became leader in 1921.

“I can only speculate why they did not let him join but one would imagine it had something to do with the fact Hitler was opinionated and they did not want to have someone there who was telling them what to do,” he told The Independent.

He added: “If he had been accepted into the German Socialist party he would have almost certainly remained on the sidelines. Hitler managed to push over the established leadership of Nazi party but that would have been very unlikely to have happened if he had joined the German Socialist party.”

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It was proposed for the Nazi party and the German Socialist party to join forces three times between 1920 and 1921 but they never chose to do so.

“It was only because of Hitler’s steadfast refusal to join the German Socialist party that the Nazi party did not,” he said. “Hitler would always hold a grudge against anyone who had crossed him and he was probably worried that again he would be pushed to the sidelines. Not only would the Nazi party be the smaller party out of the two but the guys who rejected him would be in the driving seat.”

Dr Weber argued the course of twentieth-century history was likely to have been very different had Hitler been accepted into the German Socialist party.

“It is difficult to say with certainty what had happened if he had not been shunned but it is not likely he would have been in the driving seat of the Nazi party,” he said. “There would have still been an opening for a radical right-wing party in Germany but the opening might have been exploited by a different party and therefore you would not have have triggered a world war or genocide”.

He said he was not sure why the important document which is from the testimony of Hans Georg Grassinger, the founding chairman of the German Socialist party, had never been uncovered given it has been available since 1961, adding that it had been “hidden in plain sight”.

Dr Weber told The Guardian the document records: “In the autumn of 1919, around September, Hitler appeared in the office of the publishing house to see Grassinger and offered [to] write for the paper, and to join and work for the German Socialist party. He didn’t have any money at the time and he also asked to borrow money from Grassinger. But they [told] him that they had no use for him in the paper and that they also did not want to have him in the party.”

Dr Weber’s research on Hitler will be outlined in his book Becoming Hitler: The Making of a Nazi which is due to be published by Oxford University next month.