Leaving aside the horror of the thought, the only way to stop Israel's assault on Gaza with a military force is to summon one more powerful than the IDF. There are only a few nations in the world that have a military that could take on the IDF - the US, Russia, China, I'm not sure who else. None of those countries are even remotely likely to invade Israel to stop the IDF from massacring the Palestinians. Why would they? What would be in it for them?

Even in WWII, Germany was not invaded to save the Jews from the Holocaust. That was a fortunate and welcome side-effect. But if the Nazis hadn't also invaded all their neighbours, and the Soviet Union, they could have well gone on and exterminated all the Jews in Europe unimpeded.