The first sound was a low roar; then the windows began rattling. Families woke up and looked outside to see the tanks, armoured cars, trucks and bulldozers of the British army. It was 4am on 31 July 1972, and 20,000 troops were sweeping into the IRA's "no-go zones".
The launch of Operation Motorman brought an end to "Free Derry" in the Bogside and to IRA control of parts of West Belfast. Twenty-two years later, as insurgents cause havoc across northern Iraq, most recently with the blasts in Karbala and Baghdad, US commanders are preparing for operations similar to Operation Motorman in an attempt to defeat the resistance.
I have just returned from seven weeks in Iraq, and my impression from talking to US military officers, resistance fighters and ordinary people in the Sunni Triangle is that there are two distinct threats. Most Iraqi resistance fighters will tolerate almost any attack on Americans and their local "collaborators". But they do not support the cells of mainly foreign fighters who kill civilians indiscriminately with the kinds of attacks seen on Tuesday.
Both sets of fighters, however, operate from what amount to no-go zones within many of the towns and cities of the Sunni Triangle. Although there are no Derry-style barricades, the sheer frequency of attacks and the depth of civilian hostility have led the US army in effect to pull out - and so to lose any significant ability to collect intelligence on the ground.
For example, I spent a day in Fallujah, an hour's drive west of Baghdad, without seeing a single American soldier. I also found almost universal support for the resistance. A week later, insurgents raided and captured the police station, killing 23 and freeing prisoners; the American forces made no intervention.
But now, a new rotation of more than 100,000 US troops is arriving in Iraq - trained no longer just for major combat but for a specialised counter-insurgency mission. On the streets of Ramadi, I talked to US soldiers from the 82nd Airborne, who are about to hand over to the marines. "From my perspective," said David Pit-tare, "the 82nd has been too soft. We've let ourselves be attacked and then we run away. The marines will change that. If they get attacked, they will flatten the area."
Two major lessons emerge from Ulster. The first is the need to clear any zone where a guerrilla feels safe. Duncan Spinner, whose Argyll and Sutherland Highlanders battalion came to Iraq after three years in Belfast, told me: "Above everything else, there is the need to dominate the ground - to prevent the insurgents being able to prepare and manoeuvre with ease. Only then can you get about and start gathering the sort of intelligence you need." The second lesson is the famous one about operating within the law and winning "hearts and minds". This, I was told, was emphasised by British officers who visited training camps in Texas last year to share experiences from Northern Ireland. After all, even General Sir Frank Kitson, whose manual Low-Intensity Operations caused such outrage in the early 1970s by urging army officers to prepare for counter-insurgency operations on the British mainland, stressed the need to win local consent and (unlike the Americans in Iraq) to veto the use of helicopter gunships, bombers and artillery.
Although the US is desperate to withdraw from Iraq and hand over sovereignty, and although the troops now arriving are said to be "culturally sensitised", the hardest confrontations may lie ahead.