Lawyers for the Bush administration had argued that all detainees held at Guantanamo Bay were outside the protection of the conventions because the men had been classified as "unlawful combatants".
Such a classification meant that captured terror suspects held at the US naval base in Cuba would be unable to rely on international law.
By denying them formal prisoner of war status, the US is free to interrogate them, something prohibited by the Third Convention, which requires prisoners give only their name, rank, age and number.
In a majority ruling yesterday, America's highest court disagreed with this interpretation of the men's legal status, helping to re-establish the primacy of international law over US federal law.
The Geneva Conventions are a group of international treaties designed to protect the sick or wounded, prisoners of war and civilians. They had their origin in the 19th century, when a Swiss businessman, Henri Dunant, witnessed the slaughter at the Battle of Solferino in 1859 in northern Italy, part of the struggle for Italian unification.
Mr Dunant helped treat the wounded in a church. He subsequently founded the International Committee of the Red Cross and, in 1864, the first convention for the protection of wounded soldiers was signed by 12 states.
There are four conventions, which were signed in 1949, with two additional protocols in 1977.
The First Convention was a follow-up to the 1864 agreement and protects sick and wounded soldiers. The Second extends protection to those fighting at sea. The Third, which was relevant to the judgment, covers prisoners of war and says they must be protected from harm and not prosecuted for lawful actions on the battlefield. The Fourth Convention was new in 1949 and drew on the experience of civilian suffering in Second World War. It states that civilians must not be deliberately targeted.
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