I’ve actually been trying to research this slogan and the wikipedia page is an interesting read: https://en.wikipedia.org/wiki/From_the_river_to_the_sea

To summarize it has started as a Zionist slogan that then got taken by Palestinian as an anti-apartheid meaning that finally got hijacked by terrorists including Osama Binladen. Story as old as time but it does make you wonder when do you stop and let terrorists and bad guys take our stuff.