Defence Secretary John Reid has visited some of the UK troops being deployed to Afghanistan later this year.
Mr Reid said the troops would help tackle the country's opium trade
He saw the training of some of the 3,300 extra soldiers being sent, mainly to the south, on peace-keeping duties.
He said it was "natural" in Afghanistan that insurgents would attack troops, but "we will defend ourselves".
Troops would not be involved in counter-terrorism, but would bolster security, he said, adding that it would also help the fight against UK terror.
Outlining the original purpose of the UK's presence in Afghanistan, Mr Reid said: "The reason we are doing this is to protect the British people from the sort of attacks we saw murdering thousands of people in the Twin Towers in New York.
"That was launched, prepared, trained for in Afghanistan by the terrorists, so by doing this our soldiers are protecting our interests here."
The majority of the new deployment will be sent to the south, to the volatile Helmand area, which Mr Reid admitted was "more demanding" than other regions in Afghanistan.
An extra 3,300 troops will go to the country to add to 1,100 already there and 1,950 announced earlier, but the total at any time will not top 5,700.
Mr Reid said it was a "a huge, huge challenge" and a "very, very long-term task".
"We will be there for three years but Afghanistan is not going to be Hampshire or Surrey in three years' time," he told BBC Radio 4's Today programme.
The deployment will cost £1bn over three years.
Mr Reid, who has said 90% of heroin sold in the UK comes from Afghanistan, added the presence of UK troops would allow aid workers to help opium growers develop alternative sources of income.
Mr Reid told the Commons on Thursday that he made "no apology" for sending more troops than previously expected.
He said it was not a "counter terrorism" mission but the additional support would help prevent Afghanistan from "falling back into the clutches of the Taleban".
BBC Defence Correspondent Paul Wood said: "The defence secretary himself has described southern Afghanistan as a place where the Taleban is active, the central government is weak and drugs are strong."
Mr Wood said Friday's exercise was "designed to show that the British forces will have the strength in numbers and the capabilities to meet the threat".
Shadow defence secretary Liam Fox has said his party would hold the government to account on the deployment.
"We cannot act and fail," he said.
The initial deployment would be 1,000 troops to the Headquarters Group of the Allied Rapid Reaction Corps, with the main deployment of 3,300 heading to the south, including a Provincial Construction team.
Nato's International Security Assistance Force (ISAF) currently numbers about 9,200 troops. It is expected to increase the overall number to about 15,000.
Mr Reid said it was hoped other countries - including Australia, New Zealand and the Netherlands would also send troops to strengthen the Isaf.