This past November cops lined the streets when the two teams faced each other in a World Cup qualifier playoff. Algeria won to go to their first World Cup since 1986, upsetting reigning African champs Egypt. Today they are playing in the Africa Cup semifinals. How do I know this?
Because right now every restaurant and hookah bar on my block is packed full of loud, rowdy, overzealous fans, spilling out onto the sidewalks, at times spilling into the streets and causing gridlock for blocks. There are mounted riot police at mid-block. There are uniformed officers positioned near local businesses and around the larger crowds. The local shops and restaurants all proudly display their allegiances with either the red, white, and black Egyptian flag or the green, white and red of Algeria. Some establishments (like Rotana) have both flags waving, as if saying "It's love, muthafucka, LOVE!". It is quite a spectacle to behold, but I can hear them cheering loudly OVER the episode of "LOST" I was watching on my computer, and my windows are closed.
What's ridiculous is that the rest of the year, all the Arabs in my neighborhood seem to get along well. Today they're screaming at each other from across the street.
For more info about this rivalry, here's a Sports Illustrated story about the rivalry and the aftermath of the WC qualifier, and the diplomatic tensions between the two North African nations that got so bad that Qaddafi had to step in and try to ease tensions.
http://soccernet.espn.go.com/news/story?id=731614&sec=global&cc=5901&cc=5901
Picture the Yankees-Red Sox rivalry, only if after the Yankees lost in the ALCS in '04, the state of New York severed diplomatic relations with the state of Massachusetts and urged New Yorkers in Massachusetts to return home, and the governor of Connecticut was trying to calm everyone down.