British enter Serb-held territory : D-DAY IN BOSNIA

The silhouetted shape of a field gun slung below a Royal Navy Sea King, camouflaged with tiger stripes painted over UN white, flying past the snowy mountains on a sunny winter's day in western Bosnia; the Bosnian Croat militia bowing to reality and allowing convoys to pass its checkpoints unchallenged; most of all, the sight of a Serb soldier handing a glass of home-made brandy to the crew of a Warrior.

Optimism and determination, a sense that perhaps this war is ending, fired the start of "Operation Resolute", the British contribution to Nato's peace implementation mission in Bosnia, which began at 11am yesterday in Sarajevo with the transfer of authority from the UN to the Western alliance.

The Secretary of State for Defence, Michael Portillo, visited Britain's troops yesterday, declaring in Gornji Vakuf: "This has been a historic day."

The Nato mission began amid confusion after fog blocked the arrival of the US Nato force commander, Admiral Leighton Smith. The ceremony was delayed for four hours leaving frustrated European officers complaining bitterly about a US desire to have a moment of glory.

Three hours after the Nato flag was raised, 140 troops from B Squadron Battlegroup, the Light Dragoons, crossed the front line at Brenica (renamed "Black Dog" by the British) and drove on to the village of Krupa, 13km south of Banja Luka, the Serb stronghold for so long a no-go area to Western forces.

"Bit of a proud moment," said Trooper Paul Stewart, whose Scimitar armoured vehicle was first to cross the line. Mira Vucic waved from her balcony as the troops rolled in, then ran downstairs to chat. "I think everything is going to be all right," she said. "We've had enough of war. I hope our children will be able to play and grow up in freedom." A neighbour, Srdjan Vidovic, inquired whether the British were allowed to drink; receiving an enthusiastic reply, he returned with a plastic bottle filled with grape brandy and handed it around. "It should be peace, at last," he said.

His optimism - or desperate desire - was echoed by Serb soldiers at "Black Dog", who will share control of the check-point with the 1st Battalion, the Royal Fusiliers. "We feel good - this is the end of the war," said Petar, one of a group of young soldiers watching as the Fusiliers unravelled razor wire around the check-point.

The Serb welcome was no surprise: one of the Britons' tasks is to supervise the return of Mrkonjic Grad and Sipovo to Serb control. The British - unlike their lightly armed UN predecessors - are taking no chances: three 105mm light howitzers were flown by helicopter to a position close to the front line, just in case.

Major-General Michael Jackson, commander of the British Nato sector (which will include Canadian and Pakistani troops), visited the new sites and praised the operation. "Everything is going extremely well," he said. "I very much hope there'll be no reason or need to use force at all."

The Light Dragoons were due to spend last night in Krupa patrolling the road, which winds through a steep gorge beside the River Vrbas, and making their presence felt before pushing further north.

"Operation Resolute Rat" - as it is known here because the 4th Armoured Brigade claims descent from the Desert Rats - went even better than hoped, but the British know there are 364 days of potential problems ahead. "We have a clear mission, which is to bring to an end three years of wretched war to an end," General Jackson said.

He must hope the warring parties do not misinterpret the name of their mission: "rat" in Serbo-Croat means "war".