Anti-Semitism was generally not regarded as a big problem in the Labour Party before Jeremy Corbyn's election as leader in September 2015.
Mr Corbyn and his allies on the left had spent decades campaigning for Palestinian statehood, in contrast with the more nuanced position taken by many of his predecessors.
Under his leadership, there was an influx of new members, many of whom were vocal critics of Israel and who believed the UK, along with the US, should be tougher towards Israel, especially regarding its policies towards the Palestinians and its building of settlements in the occupied territories.
Sir Keir sacked his shadow education secretary Rebecca Long-Bailey in June 2020, saying she had shared an article containing an anti-Semitic conspiracy theory.
Mrs Long-Bailey had retweeted an interview with actor and Labour supporter Maxine Peake, later saying she had not meant to endorse all aspects of the article.
In the article, Ms Peake discussed the police killing of George Floyd in Minneapolis, saying: "The tactics used by the police in America, kneeling on George Floyd's neck, that was learnt from seminars with Israeli secret services."
You can see where this is going. Also the claim from Ms Peake is factually correct.