British and American forces are teaming up with the Afghan army in a major offensive against the Taliban.It's being called one of the most defining operations since the war began nine years ago.CBS News Correspondent Rita Nissan reports from London.((PKG)(rtv video - for cbs affiliates and o&o use only)(LOCATOR: CAMP LEATHERNECK, AFGHANISTAN)NATS...AFGHAN TROOPS RUNNING WITH STRETCHERAfghan troops rush away a wounded soldier.NATS...BRITISH TROOPS RUNNING WITH STRETCHERBut these are just battle drills for now.NATS...MILITARY JEEP ROLLINGThousands of Afghan, British and American forces are preparing for Operation Moshtarak, which means TOGETHER in the Afghan language.Together... they're trying to wipe out the Taliban's last major base of power.Military leaders know it will be a tough fight.That's why they're keeping the start date .. quiet.(BRIGADIER JAMES COWAN/TASK FORCE COMMANDER) (00:26 - 00:29)THAT IS VERY MUCH A CLOSELY GUARDED SECRET.(STANDUP BRIDGE) (RITA NISSAN/LONDON) (00:30 - 00:37)BRITISH AND U-S COMMANDERS HAVE GIVEN THE ENEMY A N ULITMATUM. .PUT DOWN YOUR WEAPONS OR FACE THE WRATH OF THOUSANDS OF TROOPS.NATS... TRACTORS DRIVINGAfghan villagers are getting out of the way now.Hundreds of people piled their belongings into tractors and cars and fled their homes.NATS... MAN SPEAKINGHe says...'We left our village because of the upcoming fighting and bombardment. I want to save the lives of my children and the rest of my family.'NATS...AFGHAN TROOPSAfghan Security Forces are playing a significant role in the assault. ..but it could still be years before they are strong enough to take control..and let american and british troops return home.Rita Nissan, CBS News, London.