You're right that the violence and devastation Israel has laid upon Gaza has historic precedence. Genghis Khan completely destroyed cities that he considered cultural enemies, to give one example. You'll find many instances of genocide in the history books, even post WW2. Israel is not unique in that regard either.

But why does it matter? Does historical barbarity justify present day barbarity? It doesn't, and we all know it doesn't.

It's not the case that Israel is held to a higher standard. Russia has been widely condemned, blocked from international finance, and faces severe sanctions. On top of that, western countries have given substantial military and intelligence aid to Ukraine. Russia had reasons for their invasion of Ukraine (just like Israel has its reasons), but so what? Russia being angry at Ukraine is not a justification for destroying the country.

I also take issue with your characterization that Gaza "started it". This is a 70+ year conflict with many chapters of violence in it, and innocent Palestinians make up the bulk of the casualties. Israel is not the victim here.

Finally, it's not accurate to talk about Israel/Gaza as two nations at war. Israel controls everything coming in and going out of Gaza, electricity, water, etc. Palestine is not an independent country or a state, it's part of Israel except its citizens have no rights.