Temporary exhibition – Weapons of Mass Communication: Imperial War Museum, London

Based on the Imperial War Museum's (IWM) vast collection of 15,000 posters, which date back to 1914 and were published in Britain, Germany and Austro-Hungary, France, US, Russia and Spain, Weapons of Mass Communication essentially explores the role of advertising and publicity in governments' propaganda and policy.

In doing so, it pursues a complex historical narrative, focused on international and national politics, and presented in four chronological and largely self-explanatory sections: the First World War: Selling the War and the Peace; Interwar Europe: the Ideological Battleground; the Second World War: the Struggle for a Brave New World; and the Cold War and the New World Order.

It also explores different, and developing, attitudes to the poster itself, covering the complete gamut from, arguably, neutral information to the more authoritarian. The exhibition's first information panel explains the conflict: "Imposed upon us, they try to influence our behaviour, yet enthusiasts of the poster claim them to be the democratic medium."

After a thematic introductory section called Publicity and Propaganda, the exhibition tracks the poster's uneven transformation from a commercial advertising medium to a tool of government propaganda.

It looks at the role of the poster in the ideological battle between fascism and communism in Russia and central Europe and in the Spanish Civil War. It assesses ideologies and visions of society in Nazi Germany, occupied Europe, the US and Britain. The exhibition culminates in the use of the poster as a tool of protest - shown by responses to nuclear arms, Vietnam, Northern Ireland and Iraq.

This vast, fascinating and complex agenda is precisely what you'd hope would inform an exhibition at the IWM. But sadly it sits at odds with the way that Weapons of Mass Communication has been handled.

The exhibition contains about 290 posters, some 2 per cent of the museum's collection, together with about 25 changing examples of what are described as "e-activist" images, shown on a small electronic screen near the exit.

But despite, or perhaps precisely because of, its size, the exhibition feels claustrophobic and confusing. This impression doubtless owes something to the nature of the posters themselves. But it also reflects on the exhibition's conception, design and installation.

By definition, many of the posters are large, their images are arresting, and their crowded presentation (they are often hung three deep) means that they compete for attention in so densely packed an installation.

The sizes of the sections that make up exhibition are both very different and unpredictable. (Publicity and Propaganda, for instance, contains 36 posters whereas the First World War: Selling the War and the Peace has 106.)

But it's the 29 sub-sections that are really disorientating. Many are dedicated to particular nations - Britain and the empire, Germany, France, US, Austria and Hungary and Spain - within the exhibition's overall chronological schema.

But in the cramped exhibition spaces, they blur into each other or break-up, disappearing around corners and reappearing on different walls. It took me two visits to understand the layout.

Weapons of Mass Communication inevitably invites cross-country comparisons. Some of those are explicitly identified in various sub-sections - "good design", "selling the nation".

But other comparisons are there for the finding: the relative invisibility of ethnic minorities; the similarity of approaches by the left and the right; the repetition of particular iconography - especially apparent in the e-activists' parodies of earlier posters; various calls for civilians to contribute financially though subscriptions, war bonds, victory bonds, defence bonds, liberty loans, and to support the war effort in other ways - not wasting bread - "Mine more coal"; "Dig for victory".

Some aspects of the sloganeering remain resolutely historic, but others continue to ring true. First world war adverts persuading young men to join up and "see the world" are no different to the tactics still used, which were recently criticised in a report funded by the Joseph Rowntree Charitable Trust.

The role of women in war posters is also worth looking out for. The weaker sex is represented in various modes: shaming and otherwise urging their men to go to war, as in, "Gee!! I wish I were a man. I'd join the Navy" (US) and "Women of Britain say - 'Go'!"; persuading others to subscribe to the flag and to victory by appearing topless (France); as potentially untrustworthy as in "Keep mum, she's not so dumb!" (US); and, as fundamentally supporting the war effort themselves, as in "Women of Britain, come into the factories".

By perhaps the greatest frustration of the exhibition is not knowing what all the posters are about, or what they specifically refer to, or what particularly prompted them. Unfortunately, like many historical artefacts, relatively few of these posters speak for themselves.

"Remember Scarborough!" (1914) completely floored me, and it took a while to realise that "Remember Dec 7" referred to the Japanese raid on Pearl Harbor in 1941.

The various sections and sub-sections of the exhibition all have information panels - but that's it. But there is a stunning accompanying book, War Posters: Weapons of Mass Communication, published in association with the museum by Thames & Hudson.

This was written by James Aulich, variously described as an adviser and the exhibition's curator, who was awarded more than £300,000 from the Arts and Humanities Research Board for a collaboration between the IWM and Manchester Metropolitan University.

His three-year project was intended to catalogue and digitise around 10,000 posters from the museum's collection - in short to convert an untapped resource into an accessible one. Both the exhibition and the book complement that agenda. But it's a sad reflection that in order to understand a free exhibition, you have to buy the book.

Sara Selwood is a consultant and the professor of cultural policy and management at City University, London