The Jews always conspire against the Muslims and plot against them, as they tried to raise dispute between the Aws and the Khazraj, so Allah (swt) wanted to reassure the nation, and the companions of the Messenger of Allah (swt), at that time that the Jews:

لَن يَضُرُّوكُمْ إِلاَّ أَذًى {They will not harm you except for [some] annoyance}: What is the difference between harm and annoyance?

Harm: It has consequences, and it has an effect.

Annoyance: It goes in its time; it has no consequences.

Illustrative example: If someone hits someone with his hand, he will suffer at its time, then the trace will disappear, then this is annoyance.

But if he hits him with a stone and wounds him and his blood flows, and the trace remains, then this is harm.

So the meaning is: Whatever the Jews do, they will harm you except for annoyance, which means that the result of their plot is temporary harm that does not have a great effect.

وَإِن يُقَاتِلُوكُمْ يُوَلُّوكُمُ الأَدُبَارَ {And if they fight you, they will show you their backs;}: If they fight you, they will be defeated, and the cowardice of the Jews is well known, for they do not fight except from behind walls.

we have already said: There is specificity in the reason, but what is important is the generality of the meaning, and Allah (swt) does not date a specific event, but rather gives generality, in order to be valid for every time. For this it comes here:

 ثُمَّ لاَ يُنصَرُونَ{then they will not be aided.}: Allah (swt) made it a general law at all times, so they will never be aided, and they will not harm you except for [some] annoyance, and they will not have a decisive victory over the nation of Muhammad (saws). Likewise, the use of the word “Then” to denote the extending of time; in every battle of confrontation they will not have a decisive victory.