The goal of Operation Strike of the Sword is all of the Lower Helmand River valley.
Two British soldiers were killed on Wednesday paving the way for this latest offensive.
Waves of helicopters landed troops before dawn. About 4,000 surged forward, backed up by thousands more in one of the biggest operations in Afghanistan since the Soviet withdrawal in 1989.
In all, there are 61,000 troops from 42 countries fighting the Taliban in Afghanistan.
America’s 29,000 are due to be more than doubled by the end of the year. Britain has had a major major presence in Helmand Province since the invasion in 2001.
And the Afghan force comprises almost 90,000. The Taliban is estimated by one general to number between 10,000 and 18,000 in the south.
Pakistan has re-deployed some of its border forces to block the frontier with south-western Afghanistan to prevent insurgent fighters from fleeing.
The Taliban might be at the centre of a squeeze, but they have warned they will fight back.