This was mentioned in the Morning News, but the Reuters article we linked to sort of buried the news. The headline on the cover of today's NYT (changed slightly online) puts it more clearly: "Allies Attacking Big Taliban Haven in Afghan South."

Thousands of American, Afghan and British troops attacked the watery Taliban fortress of Marja early Saturday, moving by land and through the air to destroy the insurgency’s largest haven and begin a campaign to reassert the dominance of the Afghan government across a large arc of southern Afghanistan.

The force of about 6,000 Marines and soldiers — a majority of them Afghan — began moving into the city and environs before dawn.

As Marines and soldiers marched into the area, several hundred more swooped out of the sky in helicopters into Marja itself...

There are two good maps—of all the checkpoints, patrol bases, and main bases American/Afghan/British troops have set up today in Marja and along the Helmand River—here. (Scroll down.)

This is a complicated mission because Marja has 80,000 residents, and according to a tribal leader quoted in the NYT, 95% of those residents have decided to stick it out in their houses even though they were warned the attack was coming, meaning it's going to be hard for American/Afghan/British troops to know who is who. Already they have been "encountering intense but sporadic fighting as they began the treacherous ordeal of house-to-house searches." The plan is to root out the Taliban from Marja, seize their bomb-making and poppy-production facilities, and install an entire new municipal government and Afghan police force ("an entire Afghan civil administration, along with nearly 2,000 Afghan police officers") to protect from the Taliban from coming back. Among the American/Afghan/British troops involved in this mission, 60% of them are Afghans.

One American and one British Marine were reported killed by small-arms fire, but none from the Afghan Army, whose soldiers make up the majority of those in the fight.

The roads, walkways, and houses in Marja are believed to be packed full of hidden I.E.D.s, so many that residents have been warned (by the U.S. and by the Taliban) to stay in their houses for their own safety. "The bombs are equipped with detonators that are set off in a variety of ways: simple pressure plates, remote-control devices or wires connected to switches that are triggered by insurgents lying in wait," the Washington Post (which has more information on the I.E.D.s they've already defused and destroyed today) reports. The most dangerous part of this mission will be these next several days, military leaders agree. Here's what happened late the first day:

Late in the afternoon, insurgents and a company of Marines fought a two-hour gun battle at Marja’s northern edge. It ended when the Marines dropped a 500-pound bomb on the Taliban’s position.

After the bomb, the Marines believed that several wounded and dead Taliban fighters lay in the field in front of them. But each time they ventured into the field, Taliban fighters opened fire. After a time, the Marines decided to leave the Taliban casualties in the field...