A lot is in a name, and US and British commanders are desperate not to apply the “G” word, redolent of past quagmires, to the young Iraqi men in civilian dress who have unexpectedly been ambushing their troops in southern Iraq.

“When you are talking about classic guerrilla warfare, it generally implies a force that is accepted among its population, and we’re not seeing that here,” Brig. Gen. Vincent Brooks told a briefing at Central Command headquarters in Qatar. Wrong, says British defense analyst Paul Beaver, a former publisher of Jane’s Defense Weekly. “I believe at the moment there are very clear indications that, independent of whether the Iraqis like Saddam Hussein, they like an invasion force even less,” he said. “The Americans have a problem.”

The tactics used by Iraqi paramilitaries look more and more like guerrilla tactics every day. On Saturday, they blew up American soldiers with a car bomb near the city of Najaf. Whatever one calls them, the paramilitaries were not meant to be there. Generals have begun acknowledging that they were taken by surprise.

“The enemy we’re fighting is a bit different than the one we war-gamed against, because of these paramilitary forces,” the top US Army general on the ground in Iraq, Lt. Gen. William Wallace told the Washington Post this week. The generals should have been warned, Beaver said. “The major failure of this war so far is the Americans’ underestimation of the Fedayeen and the people’s resilience, due to an absence of tactical intelligence,” he said.

“If they had stopped to ask anybody who’d ever actually been to the place, they would know that whatever people feel about the leader, they won’t support an invasion.” US and British commanders say the surprise resistance of irregular forces will not scuttle their battle plans.

By conventional warfare measures, their Iraq campaign has been a success. Military losses on the attacking side, and Iraqi civilian casualties, are still fewer than in the 1991 Gulf War. Huge swathes of territory were taken quickly. Supply lines have proven rickety and open to disruption by paramilitaries, but fresh reinforcements are on the way.

Indeed, US and British commanders say sporadic fighting against “pockets of resistance” may look intense in the television reports of embedded correspondents, but has little strategic significance. Aides to George W. Bush say the US president finds the media’s focus on such setbacks “silly”. Most military experts are still sure that the US and British forces will be able to quickly overwhelm Iraq’s Army and force the overthrow of President Saddam Hussein as advertised.

But the appearance of paramilitaries, like the absence of jubilation among southern Iraq’s Shiites, is a sign that keeping the peace later may be more difficult than anticipated. “The Americans will win this war. The question is, will they win the peace?” said Beaver.

The deserts of southern Iraq are bad territory for guerrilla tactics, unlike the Chechen mountains or the jungles of Vietnam, he said. American and British forces will not be easy targets. But much will depend on how the US and British troops behave. Already, television pictures are showing young Iraqi men rounded up as guerrilla suspects kept behind razor wire.