Sure. Is this meant to be hard?
Anti-West stance
Corbyn hates the West. He will automatically side with any country that also hates the West and that means he would be disastrous for the defence of the country. As I have posted before, this led to him suggesting that Assad gassed his own people, that we should allow Russia to investigate whether they were behind the novichok poisoning in Salisbury, and he blamed NATO for Russia's invasion of Ukraine. Iran is anti-west, so of course Corbyn has made several appearances on Iranian government TV.
Hanging out with terrorists
His rabidly Anti-West stance means that Corbyn spends a lot of time in the company of undesirables. Famously spent a lot of time with the IRA (a file opened on him by MI5 as a potential threat to national security), inviting two people who had been in prison for IRA-related offences to the House of Commons just after the Brighton bombings. Who else did he invite to the House of Commons? His "friends" in Hamas and Hezbollah.
Here he is saying that the British government labelling Hamas as terrorists would be a big historical mistake because Hamas are just working for peace in the region https://x.com/timescorbyn/status/1724186336922386655?s=61&t=U9XrcF693-JpMxeIueYG7g
Post October 7th, who does it appear was making a big mistake with that statement? Not Corbyn of course- he was asked by Piers Morgan to label Hamas as terrorists and he refused https://x.com/piersmorgan/status/1724169911415504901?s=61&t=U9XrcF693-JpMxeIueYG7g
That's because the hard left when challenged dig in
Brexit
Anti-West Corbyn is obviously not a fan of the EU. Perhaps that was why he made such a balls-up of Brexit policy post referendum that contributed to the election defeat against Boris Johnson who at least had a clear view. https://www.channel4.com/news/factcheck/factcheck-corbyns-changing-brexit-stance
Corbyn before Party
Post-Brexit referendum he lost a no-confidence vote of his own MPs by 172-40. Did he resign, having lost the confidence of his colleagues? Of course not. He then led Labour to an election defeat against Theresa May. Did he then resign? No, he then led Labour to a crushing election defeat against Boris Johnson. Could a different leader with a strong stance on Brexit have taken on Johnson? Possibly. They could hardly have done worse (Labour's worst electoral defeat since 1935) - Corbyn was named as a big problem for Labour by many voters. Did he then resign? No, he said he wanted to stay on during a period of reflection. What a tit.
Blindspot for antisemitism
Antisemtism was a huge problem in the Labour Party under Corbyn and was poorly dealt with. Corbyn denied this, and the 130 page report detailing it. The famous anti-racist campaigner didn't support Luciana Berger, one of his own MPs who was the victim of vicious antisemitic attacks. People rightly criticise Starmer for his treatment of Rosie Duffield, but the template for the handling of Duffield was Luciana Berger. This blindspot for antisemitism is a direct consequence of his anti-West stance (which is of course automatically anti-Israel) which leads him to be around so many actively open antisemites (e.g. Hamas) that he just doesn't recognise it as a problem.
I could go on, but I think if you transplanted all of the above onto George Clooney I would still not vote for him.