The Mexican Suitcase arrives in New York

Forgotten images of a young Barrack Obama in his college days, 500 works by Howard Simmons long-abandoned in a warehouse in Chicago ('I can’t believe I forgot that I shot James Brown,' Simmons famously said on their recovery') and the Norsigian negatives - the trove of 'Ansel Adams' negatives reputedly bought for $45 at a garage sale in 2000: the re-emergence of forgotten snapshots of history always provoke excitement, intrigue and, more often than not, controversy.

But few are as comprehensive, historically important and carry such a story of miraculous survival as the 4,500 negatives contained in the 'Mexican Suitcase' - the trove of images shot by Robert Capa, David Seymour (better known as 'Chim') and Gerda Taro during the Spanish Civil War. The 'Suitcase' (in fact three cardboard boxes) was smuggled out of Paris in 1939 by Capa's assistant Csiki Weiss, after which they passed by some unknown route - legend has it they were buried in the French countryside or hidden in the basement of various South American consulates - into the hands of General Francisco Aguilar González, the Mexican ambassador to the Vichy government in the early 1940s. Unearthed in Mexico in the late 1990s by a filmmaker who inherited the collection after the death of his aunt, a friend of the General, they were finally conveyed to the International Center of Photography in 2007, where, together with magazines of the period and corresponding film footage shot by Capa, they are currently the subject of a special exhibition (on show until 9 January 2011).

That it has taken three years for the cache to be exhibited perhaps give some idea as to the scale of this record. Taken between May 1936 and spring 1939 the collection covers many important moments, including the mobilisation for the defence of Barcelona in January 1939, dynamic images of the new People's Army training in Valencia, the Navacerrada Pass on the Segovia front, and the battle of Brunete, where Taro was killed in 1937. The group also includes several rolls of portraits of Capa and Taro by Fred Stein, exiled to Paris before his flight to America, and previously unknown portraits of Ernest Hemingway, Federico Garcia Lorca, and Dolores Ibarruri (known as 'La Pasionaria).

But, as well as providing a uniquely rich view of a conflict that changed the course of European history - albeit with more than a whiff of propaganda - the collection also demonstrates how the work of these legendary photographers laid the foundation for modern war photography. Using the small 35mm cameras of the day, Capa, Taro and Chim were among the first to chronicle conflict from the nucleus of battle, as participant rather than bystander; a technique summed up by Capa's famous dictum: 'if your pictures aren't good enough, you're not close enough.' The genre could never be the same again.

That all three pioneers died on the job adds a further poignancy to the discovery of footage which, when viewed in series as contact sheets as in the ICP exhibition, has a sense of immediacy that disappears with an isolated frame: Taro was killed in a collision with a tank; Capa stepped on a landmine in Indochina in 1954 and two years later, Chim was mown down while covering the armistice of the Suez War.