By Paul Reynolds
World Affairs Correspondent, BBC News website
The arrest of the former Liberian President Charles Taylor on war crimes charges is an important step in the worldwide efforts to remove the impunity that war criminals have often enjoyed.
Mr Taylor was arrested as he tried to leave Nigeria
The international jigsaw being assembled has seen another piece fitted into place though it is still a long way from being completed.
The case illustrates two vital factors in the pursuit of war criminals. First there has to be a legal structure to charge and try them and second there has to be the political will to arrest them.
In this instance, the legal structure is the Special Court for Sierra Leone, set up by the UN and the government of Sierra Leone.
For security reasons, it will probably move from Freetown the capital to sit in The Hague, which is also headquarters for the International Criminal Court.
The Special Court brought 17 charges against Charles Taylor in March 2003. These were based on allegations that he helped direct a rebellion in Sierra Leone "as part of his continuing efforts to gain access to the mineral wealth of Sierra Leone and to destabilise the government of Sierra Leone".
The charges, which were later reduced to 11 counts, list alleged acts of terrorism, unlawful killings, sexual and physical violence, including rape, and mutilations, use of child soldiers, abductions and forced labour, and burning and pillage.
The political will for the arrest was slow in coming. Mr Taylor had taken refuge in Nigeria in 2003 but the pressure on the Nigerian government to arrest him only bore fruit on the eve of a visit to the US, and a meeting with President George W Bush, by the Nigerian President Olusegun Obasanjo.
Even then, Mr Taylor managed to escape and was arrested only as he tried to leave the country carrying bags of money.
There is no doubt that a great deal of progress has been made in tracking down war criminals in recent years.
The setting up of the special tribunals for the former Yugoslavia and Rwanda have done much to restore a feeling that justice can be done after conflicts and at least some of the perpetrators will be brought to account, though the absence of the former Bosnian Serb leaders Ratko Mladic and Radovan Karadzic speaks volumes for the lack of political will in some quarters.
Some countries have taken their own routes. Iraq itself is trying its former President Saddam Hussein. Others have preferred truth and justice commissions and have given immunity in exchange for truth and admissions.
There is a lack of political will to bring some suspects to justice
Extraditions on national or international charges are another possibility. The former Peruvian president Alberto Fujimori is currently being held in Chile while Peru's request for his transfer is considered.
General Pinochet, the former Chilean military leader, was arrested in Britain under the international torture convention but was later released on health grounds.
In the case of the former Yugoslavia, an important lesson has been learned following the death of ex-President Slobodan Milosevic in the fifth year of his trial.
The cases have to be made simpler and shorter. The Rwanda tribunal has been quite successful. It has brought justice not only to those who did the deeds but to those who ordered or allowed them.
However, the legal institutions are rather haphazard as yet and there are arguments about the best way to do this. The biggest argument is over the International Criminal Court (ICC).
The US is opposed to it but it is supported by the vast majority of other countries. US opposition was restated in the Security Council by its representative Anne Woods Patterson when the council decided to refer the situation in Darfur to the ICC in March 2005.
The US did not stop the referral but Ms Woods said the statute setting up the court was flawed and did not have sufficient protection from the possibility of politicised prosecutions.
The US favours ad hoc tribunals known as "hybrids." These combine a national and an international aspect and are usually set up under the supervision of the United Nations.
Sierra Leone is a case in point. Cambodia is another. There, former Khmer Rouge leaders are to be tried under three Cambodian and two international judges.
However these trials have been delayed and again, the will is needed to complete the process.
The ICC itself, set up in 2002, is getting going after a slow start. It has had four cases referred to it: the rebellion in northern Uganda referred by the government of Uganda in 2003, the situation in the Democratic Republic of Congo (DRC) referred by the government there in 2004 and the Central African Republic by its government and Darfur by the Security Council in 2005.
It has recently made its first arrest. This arose out of its investigation of the unrest in the DRC. The arrest was of Thomas Lubanga Dyilo, the alleged founder and leader of the Union des Patriotes Congolais (UPC), a militia accused of violence against civilians.
He is charged under Article 8 (War Crimes), specifically "enlisting and conscripting children under the age of 15 and using them to participate actively in hostilities".
But it is only a start. Amnesty International said: "This arrest and transfer of Thomas Lubanga Dyilo to the ICC...will lose its significance if more warrants of arrest are not promptly issued against other alleged perpetrators of human rights violations, including those on the side of the government and those in armed opposition groups."
No discussion of this issue could be complete without mentioning demands for the trials of President Bush and Prime Minister Tony Blair over the invasion of Iraq.
The British playwright and Nobel Prize winner Harold Pinter recently called for such trials. There is however no prospect of them taking place in the foreseeable future.