The original "axis of evil" list, pronounced by US President George W Bush in 2002, comprised Iraq, Iran and North Korea.
US Undersecretary of State John Bolton has said that Syria, Libya and Cuba are also candidates.
So what is the US case against these countries and what are its options now?
The US State Department accuses Syria of supporting terrorism and harbouring Palestinian groups it classes as terrorists.
The US has also accused Syria of playing an unhelpful role during and after the war in Iraq, allowing large numbers of armed volunteers to cross its borders to fight against the US-led coalition in Iraq.
Syria denies it supports terrorists and denies any suggestions that it has given shelter to senior members of ousted Iraqi President Saddam Hussein's former regime.
The official view in Damascus - which is shared by much of the Arab world - is that Palestinian resistance groups are not terrorists but "freedom fighters" who have a legitimate right to resist Israeli occupation of Arab land.
To punish Syria, a US Congressional committee has proposed introducing sanctions, with a bill that accuses Syria of sponsoring terrorists, occupying Lebanon with more than 20,000 troops and trying to acquire weapons of mass destruction (WMD).
Washington needs Syria, but the question is how much?
There can never be a comprehensive Arab-Israeli peace deal without Syria's signature but negotiations have all but ceased.
Since the 11 September 2001 attacks, however, Washington has found Syria to be a useful partner in combating Osama Bin Laden's al-Qaeda.
Damascus shares some intelligence with the CIA and is known to have passed on the results of at least one interrogation, that of Mohammed Haydar Zammar, a man accused of having links with al-Qaeda.
If it ever came to conflict in the future, Syria's armed forces would be no match for those of either Israel or the Pentagon.
Its large and undoubtedly tough army relies on equipment that is often more than 30 years old.
The air force is incapable of effectively defending Syrian airspace and its navy poses little threat to anyone.
However, Syria does have a number of medium and long-range surface-to-surface missiles.
Israel says it believes these missiles can be armed with both chemical and biological weapons - if that assessment is correct then this would be a powerful deterrent to any full scale military attack.
Iran's nuclear programme is worrying almost everyone now, not just the US, which has long classed the Islamic republic as "a state sponsor of terrorism".
Tehran insists its nuclear ambitions are purely civilian and that it has the right to develop its own home-grown energy programme.
But inspectors from the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), have discovered suspicious traces of enriched uranium at sites in Iran.
So Washington believes, as do several countries, that Iran is only a few years away from building a nuclear bomb.
Since the US also accuses Iran of supporting terrorists and obstructing peace in the Middle East, it is clearly worried by the situation.
On 9 October US Undersecretary of State John Bolton - a noted hawk in the administration - said: "The risk of outward Iranian proliferation of weapons of mass destruction to other countries in the region is [also] a risk we take very seriously."
America's options in Iran are limited.
The West has very limited influence over the people who take the decisions there regarding national security - they are mostly hardliners opposed to the US.
For now, hope rests with the UN's nuclear inspectors.
But if Iran fails to give them full access then, over time, other countries in the region - such as Saudi Arabia - may be tempted to try to acquire their own nuclear deterrents, thereby setting off a Middle East arms race.
If Iran were to acquire nuclear weapons, the country with most to fear would be Israel.
The last time a country in the region appeared to begin work on such weapons the Israelis carried out a pre-emptive strike.
In 1981 the Israeli air force raided and destroyed Iraq's French-built Osirak nuclear reactor.
There was widespread outrage, but it set back Saddam Hussein's nuclear ambitions by nearly 20 years.
Israel has not ruled out doing the same with Iran.
With the defeat of Saddam Hussein's regime in Baghdad, Iraq's alleged WMD capability has turned out to be far less fearsome than we were led to believe.
At the time of writing, no actual WMD have been found, despite months of searching by the coalition's 1200-strong Iraq Survey Group (ISG).
That group's interim report has been seized on by critics as proof the Iraqi threat was grossly exaggerated.
However, the report does detail extensive evidence that Saddam Hussein had not given up his ambitions to develop WMD.
So the ISG report has equally been seized on by the US and British Governments as a vindication that the former Iraqi leader was an international menace who needed to be disarmed.
The troubling question is this. If those lethal weapons really did exist at the outbreak of war in March 2003, then where are they now?
The nightmare for the West, however improbable, is that Iraq's alleged chemical and biological weapons could fall into the hands of unscrupulous terrorists linked to or inspired by al-Qaeda.
This is the hardest case of all.
The isolated, bankrupt and almost impenetrable communist state of Kim Jong-il has hinted that it may already be in a position to build several nuclear bombs.
In October 2002, US satellite reconnaissance revealed North Korea was secretly pursuing a nuclear weapons programme in contravention of a deal made with the Clinton administration.
Faced with the evidence, the North Koreans did not deny it.
They then expelled the UN's nuclear inspectors and withdrew from the Non-Proliferation Treaty (NPT).
US Undersecretary of State John Bolton has described the ruler, Kim Jong-il, as "a dictator".
In return, North Korea called him "human scum".
Regional talks mediated by China have so far failed to resolve the issue of the nuclear programme.
But there are also concerns over North Korea's long-range missile programme.
It has already tested its missiles over the northern Pacific and it has been an avid exporter of missile technology to the volatile Middle East.
Scud-type missiles deployed by Iran, Iraq, Syria and other countries are nearly all derived from North Korean know-how.
Pakistan denies the suggestion that it traded its nuclear weapons expertise for North Korean missile technology.
There are many who would argue that North Korea's dangerous and unpredictable regime was a far more worthy candidate for replacement than that of Saddam Hussein's in Iraq.
But there is a compelling reason why the US is reluctant to get into a military confrontation with North Korea.
Even without a nuclear bomb, Kim Jong-il could retaliate with enough missiles, field artillery and chemical weapons to flatten the nearby South Korean capital, Seoul.
Casualties would also be high amongst the 37,000 US troops based in South Korea, while the economic impact on neighbouring countries such as Japan could be catastrophic.
Hence nobody in the region - except possibly elements of the North Korean leadership - wants to see a conflict.
Of all the countries on the US list of so-called "rogue states", the case is the weakest for Cuba.
Like Libya, it has been ruled for decades by a now-ageing revolutionary who is tiring of the economic penalties that come with being treated as a pariah by Washington.
Fidel Castro is certainly no democrat and President Bush would dearly love to see him replaced by someone more amenable to the US.
But the accusation made last year by the US Undersecretary of State John Bolton that Cuba had a biological weapons programme was widely derided.
His boss, US Secretary of State Colin Powell, was obliged to clarify the US position, saying he did not believe Cuba had bio-weapons, merely the capability to conduct biological research for offensive purposes - a charge that could be levelled at scores of countries around the world.
In military terms Cuba presents little threat to anyone.
Although during the Cold War it sent troops to Angola, these days its armed forces are withering through underfunding.
The collapse of its former benefactor, the Soviet Union, has deprived its once powerful air force of spare parts.
At just 60,000 troops, its army has shrunk to less than a third of the size it was 10 years ago.
Fidel Castro is now approaching his 80th birthday.
The US is unlikely to want to precipitate a confrontation when change in Havana may be just around the corner.
Since the dark days of the 1988 Lockerbie bombing - for which a Libyan agent was convicted - this North African oil state has made huge strides towards being rehabilitated by the rest of the world.
It has renounced terrorism, agreed to pay compensation to the victims' families and is rejoicing in the lifting of UN sanctions.
Most dramatically, Libya has announced that it is abandoning its programmes for developing weapons of mass destruction.
It has agreed to the immediate international monitoring of its facilities, and has also promised to negotiate a new deal with the United Nation's nuclear agency and to provide guarantees on biological weapons.
Libya's leader, Colonel Muammar Gaddafi, is not the firebrand revolutionary he once portrayed himself as.
Thirty-four years in power and a bruising airstrike by the US air force on his Tripoli house in 1986 have mellowed him.
Pragmatists around him have convinced him that he needs a working relationship with the West if the country is to advance economically.
Most analysts no longer consider Libya to be a threat.