Patrick Wintour and Peter Beaumont
Britain had agreed to contribute the largest contingent of 50,000 troops to the 170,000-strong force - 'almost the entire British Army'.
The details of the operation were revealed to The Observer during a month-long investigation. The invasion was intended to last six weeks, ending before the first snows of the winter. In a series of on-the-record interviews with The Observer, senior British military figures revealed that Britain had offered the Nato Supreme Allied Commander, General Wesley Clark, 50,000 soldiers to form the core of the invasion force.
General Sir Charles Guthrie, Chief of the Defence Staff, says it would have caused Britain 'great difficulty' to contribute such a large force, but it is 'a great credit' to Britain that it had been in a position to do so.
Guthrie admits that leaving a formal Nato decision on ground troops as late as July was making it 'jolly tight'. Guthrie's deputy, Air Marshal Sir John Day, says Nato was 'within days' of making a formal decision on ground forces and expresses confidence Nato allies would have agreed.
He discloses the build-up of Nato forces around Kosovo's borders, announced on 25 May, was a 'a subtle way' of both moving to a full invasion force and cutting the time needed between a formal decision and the arrival of troops.
Day and Nato sources believe Milosevic had become aware President Bill Clinton was ready to commit ground forces when he capitulated. Other Nato sources believe Milosevic was informed of US thinking, adding that 'two or three incidents' during the war suggest vital intelligence was passing to Belgrade.
Guthrie told allies the Serb army was 'militarily overrated' and describes them as 'bully boys good at killing women, children and old people'.
Admiral Sir Ian Garnett, Director of Joint Operations, said the initial border defences would have been 'a tough nut to crack', but once inside Kosovo with flatter ground the Serbs 'would have stood little chance'. He added it would have been vital to invade before winter because tanks and soldiers would not have been able to manoeuvre 'wading through snow'.
It is also revealed for the first time that Clark fought an intensive battle with senior figures in the US administration, especially Defence Secretary William Cohen, to win approval for the ground force.
Clark appointed a secret planning team at Nato headquarters in Mons, Belgium, nicknamed the 'Jedi Knights' to prepare options for the ground force. He finally received US approval to send a heavy engineering brigade to build the key supply routes through Albania to the Kosovo border, the precondition for a land invasion.
The Jedi Knights relied heavily on British plans for a ground invasion which the MoD began drawing up from 12 June 1998. The MoD prepared six different plans of attack, including an option for the full invasion of Serbia itself.
Asked if Britain lobbied for a ground force invasion of Kosovo, Guthrie concedes: 'We were more forward in our thinking and planning than a lot of people were.'
With Downing Street's approval, Clark was given access to details of the private phone conversations between Tony Blair and Clinton to ensure he had a full understanding of the thinking of two of the most pivotal figures in the alliance.
Day also reveals that senior continental politicians, especially the Italians, privately assured Downing Street that their calls for bombing pauses were for domestic consumption and did not represent their true private views.
Clare Short, the International Development Secretary, also discloses her personal fear that she might have to leave the Government if there was a fudge forced on Britain by pressure from Nato allies.
Private papers sent to Blair at the time of the Washington summit of Nato in April (where ground troop planning was reactivated) also show, however, that General Sir Michael Jackson, British commander of the proposed Nato peacekeeping force, K-For, was in despair over the lack of direction from Nato.
Guthrie also issued a call to Britain's European partners to rethink their defence strategy. He said one of the lessons of Kosovo was that 'unless Europe does more, the Americans are going to do less.' He added: 'We could put 50,000 into the field and I do not know of another European nation that could do that, even though some of them have much bigger forces than we do.'
He claims: 'The British had a lot of influence because of the way we approached it. We had a Prime Minister, Defence Secretary, and I hope a Chief of Defence Staff, that knew what they wanted.'