Roger Waters spray painted these lyrics on the Israeli aparthied wall while visiting the West Bank city of Bethlehem on June 21, 2006, the day before he performed in the Arab/Israeli Peace Village Neve Shalom. He had originally scheduled the concert for Hayarkon Park outside Tel Aviv, but moved it after discussions with Palestinian artists and Israeli refuseniks about the Palestinian call for an international cultural boycott against Israel’s inhumane and illegal policies. He wrote about the experience in The Guardian
http://www.flickr.com/photos/darkhairedgirl/172767379/
Two years previously, Waters had helped launch the social justice organization War on Want’s campaign against the wall

Here’s the wikipedia page summarizing various positions in a more balanced way on the Israeli West Bank barrier
This may also in part be a subtle response last year’s Kanye-only-dates-white-girls-now meme.
For whatever reason, the site wouldn’t let me leave a comment on the line itself.
you still rap? You still a fucking thorn?
Sticking yourself into more, war? For what?"
Similar to the rest of the song, Jean positions herself as a thorn in the side of much commercial hip hop, causing trouble just by being herself. Her reference to getting into “war” may also be a subtle homage to Pharoahe Monch, and the title of his last album W.A.R. (We Are Renagades), which featured Jean and the track “Assassins”, which also appears on Cookies or Comas.