The United States says it has launched a major offensive against the Taliban in the southern Afghan province of Helmand.The US military says about 4,000 marines as well as 650 Afghan troops are involved, supported by Nato planes. It is the first such operation under President Barack Obama’s presidency and units moved into the Helmand River Valley earlier today. Operation River Liberty is intended to seize most of the entire Helmand River Valley – heartland of the Taliban insurgency and the world’s biggest heroin producing region. The Marines hope by appearing suddenly they can capture the Taliban’s firmest strongholds with little resistence. President Obama has declared the Taliban insurgency in Afghanistan and neighbouring Pakistan to be America’s main foreign threat.