PORT-AU-PRINCE, Haiti – Sergeant First Class Jason Jacot stood by the imposing iron gate of the headquarters of Joint Task Force Haiti, sweltering in temperatures near 100 degrees as he cradled an M-4 rifle across his body.

The sweat that trickled down his face had nothing to do with the threat of imminent death or injury from hidden explosives, suicide bombers, and snipers, as the case had been in Iraq.

For Jacot, the culprit was simply the tropical humidity.

Although the ruined streets of Haiti are filled with millions of hungry, desperate, homeless people, the 6,200 American soldiers on the ground here are facing a climate far different from Iraq and Afghanistan.

Many soldiers move unarmed or with their weapons slung behind their backs, and interaction with the public seems to be marked by a mutual, cautious respect. Still, the change in environment is dramatic for soldiers who have operated in a war zone where every alley and desert road was the front line.

But according to Major General Daniel Allyn, deputy commander of the task force, the transition from a combat to humanitarian mindset “has been far less difficult than you might think. Most of the soldiers are just happy to be able to help folks who need help.’’

About 50 percent of the US soldiers here are veterans of Iraq or Afghanistan, Allyn estimated.

“They always have the right to self-defense,’’ said Allyn, a native of Berwick, Maine. “But what our soldiers are very effective at is using minimum force to deter a situation from getting worse.’’

Staff Sergeant Stephen Roach is an example. While driving to a food distribution point, where tens of thousands of Haitians would be gathering, Roach’s driver became entangled in the chaos that passes for routine traffic conditions here.

“I hate being a traffic cop,’’ Roach snarled at an intersection with honking gridlock. He stepped out, pointed at a truck, and barked.

“You! Stop!’’ Turning, Roach pointed at a battered car. “You! Back up!’’ And to another driver, “You! Drive forward!’’

Just like that, with no arguments, the traffic began to flow.

To Jacot, this operation also is different from Iraq because the work he does to restore power to the country seems to have a permanence, or as permanent as infrastructure can be in the poorest country in the Western Hemisphere.

In Iraq, by contrast, insurgents routinely targeted repaired electrical lines in an effort to sabotage the fledgling, US-backed government after Saddam Hussein’s fall. Any major line in Iraq that was repaired would become a target almost immediately.

“I look forward to going to a place where I won’t be shot at, where the people want you there and don’t look at you as an occupier,’’ Bruce said. “I really do believe in helping people. That’s why I joined the Army.’’

Bruce, a West Point graduate from Philadelphia, served in Diyalah, Kirkuk, and Tikrit during dangerous times in Iraq. His unit, the Second Brigade of the 82d Airborne, had been scheduled to be deployed to Afghanistan later this year as part of President Obama’s troop escalation.

Now, because the 82d Airborne keeps one brigade on alert at all times for a quick-response global mission, Bruce finds himself in Haiti. He and his comrades do not know how long they will remain here.

The humanitarian nature of this mission is evident in Allyn’s daily itinerary. The major general, who commanded one of the Third Infantry Division brigades that stormed from Kuwait to Baghdad in the spring of 2003, is as likely to huddle with a civilian aid group as he is with a Navy admiral off the Haiti coast.

Late last month, Allyn met with Rear Admiral Ted Branch aboard the nuclear-powered aircraft carrier USS Carl Vinson, the first element of the US armed forces to arrive here. Ordered to Haiti hours after the Jan. 12 quake, the carrier had just begun sailing from Newport News, Va., to San Diego following four years in dock for maintenance and upgrades.

Suddenly, with new orders, the Carl Vinson picked up 19 helicopters in Florida and continued to Port-au-Prince at 30 knots.

Below decks over lunch recently, Petty Officer Third Class Luis Gray, 23, of Easthampton, Mass., talked of the satisfaction of the mission. “We’re getting help to people who need it,’’ Gray said. And high over the 4-acre flight deck, Commander Richard Jones, the carrier’s assistant air officer, took a break from the merry-go-round of arriving and departing helicopters and looked around the tower.

“When I saw the casualties come aboard, that made it personal,’’ Jones said.

The carrier treated 60 patients in the first days after the earthquake, with its medical staff working nonstop for more than 40 hours to treat a wide range of injuries, from open fractures to head injuries to lacerations.

Still, despite the good will shown by many Haitians, the soldiers realize the road from ruin to rebuilding can detour into violence any time.

Bruce said he will not let his guard down, no matter how benign the daily mission appears.

“Security will always be my primary goal,’’ Bruce said. “I owe that to my soldiers and to the people of Haiti.’’