On Monday, the presiding military judge at the Guantanamo tribunals, Col Peter Brownback, delivered a bombshell when he dismissed all charges against one of two detainees.

Col Brownback ruled that tribunals could only try those deemed to be "unlawful enemy combatants". The concept of "unlawful combatants" comes from the second world war, in which German saboteurs were caught in the US wearing civilian clothes. They were executed after courts held that they were not subject to the protection of the Geneva conventions - which say that civilians can become combatants but must carry arms openly and respect the rules of war.

George Bush maintains that the detainees are not normal combatants because of their alleged links to al-Qaeda.

According to the administration, some prisoners in Guantanamo may never be granted a trial, either military or civil, and may be detained until the end of the war on terrorism.

Who are these two men and where are they from?

One is Canadian Omar Khadr, who was 15 when he was captured in Afghanistan five years ago. He is accused of killing a US soldier with a grenade and of wounding another during a battle at an al-Qaeda compound in 2002. The other is a Yemeni detainee, Salim Ahmed Hamdan, who is accused of being Osama Bin Laden's driver. Charges against him were also dismissed.

Those held in Guantanamo are not, for the most part, the top al-Qaeda prisoners being held by the US. These are in detention in so-called CIA "black sites" scattered around the world. When, shortly after 11 September 2001, Bush gave a military order to detain non-citizens suspected of having ties with al-Qaeda or other terrorists groups, hundreds of socalled "enemy combatants" were rounded up and taken to prisons in Guantanamo Bay.

Most of the detainees are not, in fact, the hardened terrorists the Pentagon often claims it is holding. A majority of the detainees are not even Afghans and were not even picked up by US forces. The majority are from Arab countries, with many arrested in Pakistan by Pakistani authorities. At least three were picked off Pakistani buses in apparently random sweeps for foreigners.

Why, despite the criticisms, is Guantanamo Bay still in operation?

Guantanamo has been called George Bush's 21st-century Gulag and the detention camp remains the centrepiece of the US "war on terror". It endures, despite scandals about the torture of prisoners during interrogation, hunger strikes by detainees and a US Supreme Court decision in 2004 allowing prisoners to challenge their detention in the US federal courts.

Although the system is now a shambles, one official said the decisions revealed that the military commissions "are not pliant or tame tools of the state".

The Pentagon paints a rosy picture of the prison, saying the inmates receive free medical, dental and psychiatric care - something 47 million Americans do not have. They can amuse themselves playing "basketball, soccer, ping-pong and board games". In practice, ex-detainees say the place is a living hell that drives many to attempt suicide.

Guards monitor them round the clock, harsh lights are on constantly and everywhere there are metal surfaces that amplify any noise to the point where it is unbearable.

Why can't the detainees be tried in the normal way rather than by these tribunals?

The US divides its detainees into two groups, those who took part in hostilities against the US and its allies, and those it says were "associated" with al-Qaeda or the Taliban. In both cases, it claims they are too dangerous to be tried in a normal court with rules of evidence, an appeals process and the right to a lawyer of their choice.

Four years ago, Bush declared that the US would not be bound by any part of the Geneva treaties in dealing with prisoners in the fight against terrorism. Instead, he ordered that US forces treat captives in ways "consistent" with the conventions.

The Pentagon says the treatment of the detainees is consistent with international laws of war. It says that defendants will be able to consult civilian lawyers, and that there will be media coverage, so they will not be in secret. However, the defence lawyers are military officers handpicked by the Pentagon. Parts of the trials can be held in secret. Acquitted suspects may continue to be held for security reasons. This is the case of the two men whose charges were dismissed on Monday.

So why has the US government taken so long to deal with the detainees?

The US wants to keep the detainees out of circulation as long as possible and it points out that at least seven of the 146 it has released so far have returned to fight, despite signing pledges to renounce violence. Two are believed to have died in fighting in Afghanistan, a third was recaptured during a raid on a suspected training camp. Of the prisoners the US believes no longer pose a threat, 57 have been transferred home: 29 to Pakistan, seven to Russia, five each to Morocco and the UK, four each to France and Saudi Arabia, and one each to Spain, Sweden, and Denmark.

How many are still in Guantanamo and how many have got out?

There are about 500 prisoners in US captivity, although the US defence department routinely rejects requests for information on all of the detainees held at Guantanamo Bay. Thanks to a habeas-corpus petition filed by volunteer lawyers representing Guantanamo detainees, the US had to file court documents on 132 of the detainees, or around a quarter of the prison's population.

What happens next?

The military tribunals have been thrown into confusion by the rulings on Khadr and Hamdan. It is doubly embarrassing for the British government, which has refused to accept two British residents being held in Guantanamo back into the country, following years of captivity. Critics say the military tribunals have now been revealed to lack international legitimacy and legal authority. One of the most criticised features of the system is the "combatant status" panel, which decides whether a detainee should go to trial. But the prisoners are not allowed a lawyer at this stage and cannot see much of the evidence presented against them. At times the Pentagon has interfered to over-rule decisions it did not like. The whole system set up by Congress last year may need to be revamped.