In an army base in Baghdad, in functional wooden booths in a white-walled room, a row of young men in uniform stare at computer screens. Many are emailing, instant messaging or playing online card games with their wives and girlfriends seven or more time zones away. There is a background hum from others talking on a bank of phones. One soldier can be heard protesting: 'You have no idea what I'm going through out here.'

With the Iraq war in its sixth year, some of these American soldiers are on their third or fourth combat tour - 15 months away from home with just 18 days' leave. The strain is showing on their relationships and many will return home, exhausted, to find a disenchanted wife has walked out. Divorce rates among the US military are soaring.

Corporal Leonard Allen, 33, is missing his son's first year of life. A member of the 2-4 Infantry 'Warrior' Battalion, 10th Mountain Division, Allen served a nine-month stint in Afghanistan in 2006. Normally he could then have expected at least a year at home. But eight months later he and his comrades were training in Kuwait, then deploying for a long tour in Baghdad.

'There were a lot of deployment babies after Afghanistan,' Allen joked. His son Colton is eight months old. 'I've seen two and a half months of his life. My wife Andrea gives me daily progress reports - he's learning to crawl - but it's a shame when a father has to miss being there. Six or nine months here wouldn't be so bad, but these 15-month tours are killing everybody.'

Allen, a former bill collector now regularly on patrol in the streets of Baghdad, married two years ago in Las Vegas. 'We knew there was a chance I'd be sent to Iraq. She was pretty down for a while, quite sad, and she worries about me here. She knows why I'm here and she's glad, but she wants me to come home.'

Andrea, 33, keeps in touch with her husband via the internet. In an email from their home at Fort Polk, Louisiana, she told The Observer: 'I miss him every minute of every day. It helps to think about that first hug I'll get when I see him next and all the things we'll do when he comes home.

'When he left for this deployment, Colton was only two and a half months old. It's so hard that my husband is missing most of our son's "firsts" and that Colton only gets to see his daddy on pictures or videos. I tell Leonard about everything and send him tons of pictures, but a picture can't capture everything, like Colton's funny sense of humour or how sweet it is to listen to him babbling in his crib when he's waking up in the morning. Fifteen months is just way too long.'

Iraq, which has seen husbands return home with mental scars or missing limbs, preys on her mind. 'I worry about him all the time; even if I'm not consciously thinking about it, the fear is always there. Any time someone rings the doorbell, my heart starts beating faster and I say a prayer that it's not someone coming to tell me something happened to him.'

The couple have rules for their long-distance marriage. 'It's impossible to explain how hard it is to be separated from your spouse for such a long time. You really need to have good communication and a strong commitment to make it work. We're really careful to never actually fight on the phone, but if we have any kind of disagreement we make sure it's resolved before we hang up, and always end our calls with an "I love you".'

The pressure of deployments in Afghanistan and Iraq have become too much for some. There were about 8,700 divorces involving American soldiers last year, compared to an estimated 5,500 in 2001. Research shows that career soldiers are much more likely to contemplate divorce than in the past.

The anguish is exemplified by John Callahan, 42, a corporal who was away from home for nearly two years. In November 2006, Callahan's machine-gun malfunctioned during a firefight, wounding him in the groin and leg. Recovering from an operation in hospital, he spoke to his wife by phone and could hear a male voice in the background. 'Haven't you told him it's over?', said his wife's boyfriend. 'That you aren't wearing his wedding ring any more?'

Some couples face reunions in which the returning soldier has been disfigured, paralysed or lost one or more limbs or suffered a life-changing brain injury. US marine Ty Ziegel suffered horrific burns in a suicide bomb attack in Iraq but returned home and married his childhood sweetheart - only to separate just before their first wedding anniversary.

Last year suicides among military personnel reached their highest level since 1990 when 108 soldiers took their lives, compared with 102 in 2006 and 85 in 2005. Combat stress is growing, with one study finding that one in five personnel returns from Iraq and Afghanistan with symptoms of post-traumatic stress disorder or major depression.

Unlike during the Second World War and the Vietnam War, when many more people were directly touched by the conflict, today's military families can feel isolated from the rest of society. Separation can be particularly difficult for young couples with little worldly experience. Specialist Chris Haun, 21, from Louisville, Kentucky, married his girlfriend, Jess, just before he was deployed to Baghdad, but said they have already hit problems. 'We've had many nights when we wanted to separate,' he said. 'We've had downs as well as ups and we've talked about divorce.'

He continued: 'She was a very dependent woman and now she's becoming more independent. She is 20 and in college and getting ready to hit that age with new freedoms, new friends and alcohol. It's a time of life when things change. It's a day-to-day game. You always have the question in your head, "What's going on?", but that's where the trust comes in.'

The US army has sought to address the issue with counselling and chaplain-led programmes such as Strong Bonds, which includes weekend retreats for couples. At military bases there are television slots and leaflets offering advice on 'How to keep a marriage strong during a deployment', with tips including 'Read a relationship book together', 'Communicate', 'Pray for each other' and 'Avoid arguments'.

James Pritchard, a chaplain at Loyalty base in east Baghdad, said that 38 soldiers had come to him to discuss marital problems since the start of the year. 'Probably 10 of them had found out or got evidence that their wife was leaving them or seeing somebody else,' he said.

'It's a big issue, especially with younger soldiers who've married somebody they haven't known very long. They suddenly have extra money coming in and the lifestyle of the spouse at home lends itself to extra-marital affairs. We've had soldiers go home and find the house empty, the wife and kids gone.'

He added: 'I tell soldiers that, if she was a topless dancer before you met her, when you come home don't be surprised if she's no longer your wife.'

Sergeant Larry Driscoll, 42, who has been married twice, said: 'One deployment has a profound effect on a marriage. If the marriage is on shaky ground, you think distance will make it better, but it exacerbates the problems three times over. To get through a second or third deployment, that's a miracle.'