Exhibition explores sexuality during times of conflict. From Hugh Schofield in Paris

SINCE the face of Helen launched a thousand ships against Troy, sex and warfare have been intimate companions. Now the way the two world conflicts of the last century affected the private lives of millions is the subject of a hit exhibition in Paris.

The show, Loves, Wars and Sexuality 1914-1945, in the normally staid surroundings of the army museum at the Invalides military complex - draws on hundreds of documents, posters and photographs to examine how society, the state and the individual deal with sex during periods of "total war".

"Our aim is to help understand sexuality better by looking at it through the prism of war, and understand war better through the prism of sexuality," said curator Fabrice Virgili. "In war rules change. People are thrown together in circumstances that would never normally occur. Relationships are flung into disarray. And at the end the way men and women interact is totally different."

During both wars, governments used eroticism to encourage recruitment and beef up martial spirit. Posters of bare-breasted women urging no surrender in 1914 France are juxtaposed with images of muscled American sailors loading phallic missiles in the second world war.

The pain of separation is evoked in first world war "trench art" created by troops using shell cases and other refuse. One diptych etched on bits of wood sets the "clinch of war" - two men locked in mortal combat - against the "clinch of love" on the soldier's dreamed-of return.

Soldiers had little in the way of what we would class as pornography - the few explicit items on display belonged to wealthy collectors - but they exorcised their lust on magazine pin-ups and fantasies of their own devising. For example, the inside of a German blockhouse on the French Atlantic coast bears a life-size mural of an ecstatic bare-breasted woman in a shower. In an extraordinary snapshot from the first world war, a north African French soldier lies beside a sand sculpture of a naked girl on a Belgian beach.

"The nearness of death and the acceleration of time encourage a yearning for passion, pleasure, even transgression," said historian Antoine de Baecque. "But at the same time the authorities need to regulate the sexuality of their troops, because it is now a matter of state interest."

For governments at war, sex had to be controlled. Mishandled, it led to indiscipline, disease, spy scandals. Handled carefully, it meant obedience. Handled malevolently, it was a tool of war itself: rape has always been a right of conquest, and the world wars were no exception.

Bordellos were set up for soldiers, and French troops got "military campaign brothels" in 1918 so local girls would not be "contaminated". Later, Germans stationed in Paris during the second world war received a list of authorised "maisons closes"- brothels - and a list of clinics in case of infection.

The exhibition includes a rare example of American condoms from the time of the first world war - not so different from today's - and pictures of disease-ridden organs. In a 1916 photograph, Senegalese auxiliaries bare their buttocks for mercury injections to combat syphilis.

As for sexual violence, Russian posters whip up anti-German sentiment with stylised images of ravished Soviet maidens. But the reality - far more unsettling - is contained in a series of small black and white photos of a rape found on a German prisoner. They are the only graphic depiction of the sexual act in the exhibition, and their impact is horrifying.

For the museum, behind the Invalides church where Napoleon is buried, the exhibition is a departure from its usual displays of uniforms and weaponry. But its director, General Robert Bresse - a veteran of the Balkan wars - says the theme is essential.

"From my experiences commanding 1000 men and a few young women in the Balkan inferno, I know well that naivety in the field of sexuality is fatal," he said.

"Separation from loved ones, the sensation of danger, biological urges: all combine to form an explosive cocktail which it is the responsibility of every commanding officer to control."