Poetry, wrote W.H. Auden, makes nothing happen. This may or may not be true, and making things happen may or may not be something poetry should even aspire to. But this didn't stop Auden journeying to bear witness to the horrors of war throughout the 1930s – first in Spain, then in China – and it didn't stop him writing about what he saw.

Martha Gellhorn, travel writer, novelist, and one of the greatest war correspondents of the 20th century, made similar journeys, to Spain and China, but also to Dachau, to Vietnam, to Latin America, writing about every major conflict of her lifetime. She only stopped when she was too old to carry on ('you have to be nimble'). Writing and bearing witness, almost always intertwined, were at the centre of her extraordinary life, and she changed many things: US domestic policy during the depression, the nature of war reportage and, perhaps most crucially, what it was possible for a woman to achieve.

Gellhorn felt a profound need to take the side of the dispossessed, and did this via her writing. In the 1930s she travelled with her then husband, Ernest Hemingway, but he, frustrated by her professional dedication and outshone by her brilliance, felt increasingly alienated from her: 'Are you a war correspondent or a wife in my bed?' he asked her petulantly. 'A war correspondent', was probably her unthinking reply, and the couple later parted company.

Determined, quick-witted and unbelievably brave, certain episodes from her life read like comic-strip action hero scenarios: stripping naked to discombobulate a Nazi officer and avoid arrest; impersonating a stretcher bearer to experience the D-day landings first-hand. She was also filled with gratitude. Looking back over her life she admitted: 'I'm overprivileged. I've had a wonderful life. I didn't deserve it but I've had it.'