Nearly 50 years before the war in Iraq, Britain and America sought a secretive "regime change" in another Arab country they accused of spreading terror and threatening the west's oil supplies, by planning the invasion of Syria and the assassination of leading figures.

Newly discovered documents show how in 1957 Harold Macmillan and President Dwight Eisenhower approved a CIA-MI6 plan to stage fake border incidents as an excuse for an invasion by Syria's pro-western neighbours, and then to "eliminate" the most influential triumvirate in Damascus.

The plans, frighteningly frank in their discussion, were discovered in the private papers of Duncan Sandys, Mr Macmillan's defence secretary, by Matthew Jones, a reader in international history at Royal Holloway, University of London.

Although historians know that intelligence services had sought to topple the Syrian regime in the autumn of 1957, this is the first time any document has been found showing that the assassination of three leading figures was at the heart of the scheme. In the document drawn up by a top secret and high-level working group that met in Washington in September 1957, Mr Macmillan and President Eisenhower were left in no doubt about the need to assassinate the top men in Damascus.

Part of the "preferred plan" reads: "In order to facilitate the action of liberative forces, reduce the capabilities of the Syrian regime to organise and direct its military actions, to hold losses and destruction to a minimum, and to bring about desired results in the shortest possible time, a special effort should be made to eliminate certain key individuals. Their removal should be accomplished early in the course of the uprising and intervention and in the light of circumstances existing at the time."

The document, approved by London and Washington, named three men: Abd al-Hamid Sarraj, head of Syrian military intelligence; Afif al-Bizri, chief of the Syrian general staff; and Khalid Bakdash, leader of the Syrian Communist party.

For a prime minister who had largely come to power on the back of Anthony Eden's disastrous antics in Suez just a year before, Mr Macmillan was remarkably bellicose. He described it in his diary as "a most formidable report". Secrecy was so great, Mr Macmillan ordered the plan withheld even from British chiefs of staff, because of their tendency "to chatter".

Concern about the increasingly anti-western and pro-Soviet sympathies of Syria had been growing in Downing Street and the White House since the overthrow of the conservative military regime of Colonel Adib Shishakli by an alliance of Ba'ath party and Communist party politicians and their allies in the Syrian army, in 1954.

Driving the call for action was the CIA's Middle East chief Kermit Roosevelt, grandson of former president Theodore Roosevelt. He identified Colonel Sarraj, General al-Bizri and Mr Bakdash as the real power behind a figurehead president. The triumvirate had moved even closer to Nikita Khrushchev's orbit after the previous year's disastrous attempt by Britain and France, in collusion with Israel, to reverse the nationalisation of the Suez canal.

By 1957, despite America's opposition to the Suez move, President Eisenhower felt he could no longer ignore the danger of Syria becoming a centre for Moscow to spread communism throughout the Middle East. He and Mr Macmillan feared Syria would destabilise pro-western neighbours by exporting terrorism and encouraging internal dissent. More importantly, Syria also had control of one of the main oil arteries of the Middle East, the pipeline which connected pro-western Iraq's oilfields to Turkey.

The "preferred plan"adds: "Once a political decision is reached to proceed with internal disturbances in Syria, CIA is prepared, and SIS [MI6] will attempt, to mount minor sabotage and coup de main incidents within Syria, working through contacts with individuals.

"The two services should consult, as appropriate, to avoid any overlapping or interference with each other's activities... Incidents should not be concentrated in Damascus; the operation should not be overdone; and to the extent possible care should be taken to avoid causing key leaders of the Syrian regime to take additional personal protection measures."

Sabotage

The report said that once the necessary degree of fear had been created, frontier incidents and border clashes would be staged to provide a pretext for Iraqi and Jordanian military intervention. Syria had to be "made to appear as the sponsor of plots, sabotage and violence directed against neighbouring governments," the report says. "CIA and SIS should use their capabilities in both the psychological and action fields to augment tension." That meant operations in Jordan, Iraq, and Lebanon, taking the form of "sabotage, national conspiracies and various strong-arm activities" to be blamed on Damascus.

The plan called for funding of a "Free Syria Committee", and the arming of "political factions with paramilitary or other actionist capabilities" within Syria. The CIA and MI6 would instigate internal uprisings, for instance by the Druze in the south, help to free political prisoners held in the Mezze prison, and stir up the Muslim Brotherhood in Damascus.

The planners envisaged replacing the Ba'ath/Communist regime with one that was firmly anti-Soviet, but they conceded that this would not be popular and "would probably need to rely first upon repressive measures and arbitrary exercise of power".

The plan was never used, chiefly because Syria's Arab neighbours could not be persuaded to take action and an attack from Turkey alone was thought to be unacceptable. The following year, the Ba'athists moved against their Communist former allies and took Syria into a federation with Gen Nasser's Egypt, which lasted until 1963.