At the beginning of his 19th-century treatise On War, the German military theorist von Clausewitz referred, dismissively, to “certain self-imposed, imperceptible limitations hardly worth mentioning, known as international law and custom”.

In the heat of the fighting in Iraq, it is worth pausing to reflect on the important role now played by international law in governing the conduct of war.

Armed conflict has long been tempered by basic humanitarian principles. After the Second World War, the four Geneva Conventions of 1949 to 1950 strengthened the protection to be given to civilians, prisoners of war, and the wounded and sick members of armed forces, whether in the field or at sea.

As Adam Roberts and Richard Guelff point out in their Documents on the Laws of War (Oxford University Press), these agreements contain “manifest imperfections”.

Ambiguous legal concepts are open to abuse. Technological developments have made the scope of the Geneva Conventions inadequate. Changes in the nature of warfare have made less relevant some of the fundamental legal distinctions: between war and other forms of armed conflict, and so between armed forces and civilians.

Yet there can be no doubt that, for all its inadequacies, international law has a positive effect in mitigating the horrors of war. With rare exceptions, states do comply, whether for domestic political reasons, for propaganda purposes, to encourage proper treatment for their own soldiers, or for fear of war crimes tribunals. When we are involved in a war with a particularly ruthless enemy (one prepared to abuse prisoners of war and parade the bodies of coalition soldiers for the cameras), it is vital to emphasise the basic principles which ought to govern our conduct.

First, there are limits to what a state may do to advance its strategic aims. The status of prisoners of war must be respected, civilians taking no part in hostilities must be treated humanely, and we may not torture any person to obtain information.

Secondly, these limits apply whatever the political or strategic considerations to the contrary. As Article 1 of each of the Geneva Conventions asserts, states undertake to respect their legal obligations “in all circumstances”. This will make war aims harder to achieve and it will endanger the lives of our forces. Some distinctly unpleasant people will benefit. But, as Justice Frankfurter observed in the United States Supreme Court in 1950: “It is a fair summary of history to say that the safeguards of liberty have frequently been forged in controversies involving not very nice people.”

Thirdly, it is inevitable (however regrettable) that in wartime, civilians will be killed and mistakes will be made. But there is a vital difference between armed forces that seek to comply with international law and those who deliberately defy legal requirements. As Justice Wendell Holmes commented in The Common Law (1881), “even a dog distinguishes between being stumbled over and being kicked”.

Fourthly, victory, when it comes, imposes continuing obligations to treat the defeated enemy with respect, however badly it may have behaved.

Rabbinic commentaries on the biblical story of the Children of Israel fleeing across the Red Sea suggest that when the pursuing Egyptian soldiers drowned, God was angry with the angels in Heaven because they were singing with joy. God explained that they should have been sad that some of his creatures had been killed.

Fifthly, defeated enemies are entitled to their legal rights and to due process of law. In his 1946 diaries, the politician and diplomat Sir Harold Nicolson recorded a visit to the Nuremberg war crimes tribunal. A film was being shown of a Nazi trial, where the judge was screaming abuse at the defendant. When the film ended, and the lights went on, the presiding judge politely asked the defence counsel, Dr Rudolf Dix (the leader of the Berlin Bar), to “please continue” his examination of the witness. Nicolson observed that “the contrast between violence and calm” was so striking that “even the defendants moved uneasily”.

These are all applications of the fundamental principle recognised and asserted by Aharon Barak, the president of the Supreme Court of Israel, when ruling in 1999 that it was unlawful for the State to torture terrorist suspects to obtain information to protect citizens from further attacks. It is a defining feature of a liberal democracy that “not all means are acceptable to it, and not all practices employed by its enemies are open before it”. Our society must “often fight with one hand tied behind its back”.

Such self-imposed limitations will undoubtedly delay the defeat of Saddam Hussein. But they are essential to the moral and political force of the case for removing from power in Iraq a dictator who has repeatedly violated basic rights and obligations.

The author is a barrister at Blackstone Chambers in the Temple and a Fellow of All Souls College, Oxford.

Explore Law

• Columnists

• Law Panel

• Law Reports

• Student Law

Times Recommends

• Legal school 'earned £1m by exploiting bar course'
• Axe may fall on 5,000 more City lawyers
• Prince Charles named in £81m court battle