When the pro-Palestinian marchers came round the corner of the Strand in London, drums beating and megaphones blaring, they saw a row of Israeli flags. The boos were loud and there were plenty of obscene gestures. Both sides chanted "shame on you" at each other, with the police standing between them. The pro-Palestinian protesters shouted "From the River to the Sea", along with the other chants. Those who defend the slogan say it is a simple call for freedom. But it is understood by others to invoke the destruction of Israel - and now it was aimed at those bearing...