Lucifer Sam
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It is really interesting that when the USA and the UK invaded Iraq - and they stopped counting the bodies - the left was not calling it genocide. The left screamed 'illegal war'.
Russia has invaded Ukraine. I don't see the left calling it genocide in Ukraine. I don't see the left marching every week or taking over university campuses to condemn the genocide in China. Or the genocide in Syria. Or the act of genocide that took place on 7th October. Instead, it's Hamas has the right to attack the invader.
The USA and the UK bomb the Houthis. No one shouts genocide.
But when Israel invades Gaza - let's throw the word 'genocide' at them, over and over again, and really rub their Jewish faces in it.
I would agree with you that 20 or 30,000 people killed out of a population of 2 million is hardly much of a genocide. Probably "ethnic cleansing" is a more accurate term for the Israeli mission in Gaza, since they basically want it uninhabited or scarcely inhabited or settled by Israelis. Whether the Gazans die or leave, Israel will be satisfied as long as they're mostly gone—or else living there in such extreme squalor and malnutrition that it really would be the kind of "open air concentration camp" it has sometimes been described as.
Given the context of Israel's history, you should be able to appreciate why "genocide" is a more fitting shorthand for Israel/Gaza than it is with many of the instances you mentioned above.