United States marines have been told by their commander they are making history as they try to wrest control of Afghanistan's southern Helmand province from the Taliban.
The US military says about 4000 Marines and 650 Afghan personnel have met little resistance so far.
The BBC reports the aim of the offensive is to provide security ahead of presidential elections this August.
It is part of a major push to defeat the Taliban and al Qaeda.
The mission began early on Thursday and is named Operation Khanjar, meaning "Strike of the Sword".
Southern Afghanistan is considered a Taliban stronghold. Helmand has seen the worst violence of anywhere in Afghanistan.