The world inhabited by the likes of Osama Bin Laden is a "mean, nasty, dangerous, dirty" place according to US Vice President Dick Cheney.

If America's security agencies are to halt the man accused of the World Trade Center and Pentagon attacks, they too must put certain niceties aside he has argued.

Bloodletting in Vietnam saw the CIA reined in

"Everything is under review," agreed Secretary of State Colin Powell, which some have interpreted as a willingness to allow the CIA to ignore a long-standing ban on assassinations.

Amid a wave of public revulsion at so-called "covert" operations directed by the CIA against foreign leaders deemed threatening to the United States, President Gerald Ford issued Executive Order 12333.

For 25 years it has decreed that "no person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination".

President Ford was reacting to Senate and House committee reports both concluding that the CIA had become a "rogue elephant" crushing foreign citizens under foot in its bid to win the Cold War.

Gone rogue

For instance, more than 20,000 Vietnamese were killed during the CIA-guided Operation Phoenix intended to weed out communist "agents" from South Vietnam.

Rather than delivering victory, such bloodletting gave added moral momentum to those Americans protesting about their country's role in the Vietnam war.

However, the agency's operations in the 1960s and 1970s included not merely the bloody, but the downright outlandish too.

Not everyone was so keen on Castro

Operation Mongoose sought to eliminate Cuban leader Fidel Castro with a number of unlikely weapons.

In 1963, the CIA cooked up a plot to off the cigar-smoking communist with an explosives-packed cheroot.

An exploding seashell was also to be placed temptingly close to Castro's favourite scuba-diving spot.

Again exploiting the revolutionary's aquatic hobby, US agents were hoping to make him a gift of a deadly "contaminated" wet suit.

While Castro survived these (and other less comic) attempts on his life, others were less fortunate.

Coup backers

The family of Chilean general Rene Schneider is currently suing former Secretary of State Henry Kissinger and former CIA boss Richard Helms for their alleged involvement in a bungled 1970 kidnapping which left the soldier dead.

Schneider had opposed any military coup aimed to deny socialist Salvador Allende the Chilean presidency. Allende was later killed in just such a CIA-backed rebellion, led by General Pinochet.

The CIA were accused of meddling in Chile's bloody coup

Though Executive Order 12333 outlawed "assassination", Janes Intelligence Review says the CIA continues to initiate operations which "indirectly" jeopardise the lives of foreign adversaries - most notably Iraq's Saddam Hussein.

Critics have said the CIA's support for coup organisers and rebel groups keen to eliminate figures such as Saddam makes a mockery of 12333.

However, charges of hypocrisy are hard to assess because the order fails to define what "assassination" actually is, says Dr Kevin O'Brien, an expert on intelligence and asymmetric warfare at the Rand Institute.

Actively seeking the death of Osama Bin Laden may not constitute "assassination", rather the "decapitation of a military command structure".

All's fair in war

"Because the United States has effectively declared war, this is a situation of conflict and Bin Laden is a legitimate target under international law," says Dr O'Brien.

However, actually killing the al-Qaeda leader may only serve to make a martyr of him and win the terror network willing new recruits.

Bin Laden is thought to have assassinated the Taleban's rival Ahmed Shah Masood (L)

Plotting to kill Bin Laden and failing - as President Clinton says happened while he was in office - may also strengthen his hand.

Fidel Castro, still ensconced in power in Cuba having seen ten US presidents come and go, revels in the kudos of having survived a supposed 637 assassination attempts.

"I don't feel worthy of such a high honour," he once said.