For every US soldier or Marine deployed in harm's way overseas, three remain in the US working to support the mission. But, as the BBC's Katie Connolly finds, missing the defining deployments of a military era can be difficult to come to terms with.

When Jay Agg signed up for the US Marine Corps after the 9/11 attacks, he knew he risked being severely injured, perhaps losing limbs. He knew he might even lose his life.

Yet, much to the surprise of many civilians, when his service came to an end in 2006 and he hadn't served in a combat zone, Mr Agg was sorely disappointed.

Some might imagine that soldiers who don't get deployed breathe an enormous sigh of relief, pleased that they are remaining on safe soil, far from danger.

But that is rarely the case - and it's one more reason why many servicemen and women feel deeply misunderstood.

In the Vietnam era, dodging the military draft wasn't uncommon. Young men fabricated injuries, rushed into marriages or moved to Canada to avoid fighting and possibly dying in the bloody Asian conflict.

But in an all-volunteer military, those who sign up are steeled for their possible fate, so missing out on a battlefield tour can be a source of frustration, disgruntlement and, for some, shame.

Many in the armed forces feel that too few civilians fully appreciate the drive to serve in combat.

"The root cause of the misunderstanding is that the average person wouldn't actually want to volunteer for the military, so they don't understand that motivation to fight in war zone in the first place," Mr Agg, who now works as the national communications manager for veteran's group Amvets, told the BBC.

Fighting instinct

In almost a decade of engagements which have claimed the lives of some 5,500 US troops in Iraq and Afghanistan, a large proportion of the US Army and Marine Corps has been sent into combat zones.

Image caption Marines say they are aware of the risks when they sign up for service

But for every individual Marine or soldier deployed, three remain in the continental US, working to support the mission.

For many, being left behind is difficult. The instinct to fight is powerful, driven partly by camaraderie and partly by training.

Some liken the experience to football players who have trained hard, honing their skills, but never get to play a match. The irritation is palpable.

Moreover, it's hard for those who remain behind to explain to friends and acquaintances why remaining on a US base is such a blow.

'So what did you do then?'

Itzak Lefler, who joined the Marines after graduating from high school in 2001, says that when people find out he was a Marine they almost invariably ask him if he's killed anyone.

US military by the numbers There are 1.47m members of the US Armed Forces, which is less than 1% of the US population

The Army is the biggest branch with 560,000 soldiers, followed by the Air Force (336k), Navy (330k), Marines (210k) and Coast Guard (43k)

Some 140,100 servicemembers are in Iraq, about 75% of whom are Army or Marines

About 20% of the total armed forces are currently serving in Iraq or Afghanistan

A further 301,000 are serving in other foreign posts

It's a complicated and difficult question for all servicemembers, even those like Mr Lefler who, despite his hopes, did not see combat.

"It's always hard for me to tell people I wasn't deployed. Conversations usually end up with "so what did you do then?" as if I had no purpose in the military if I wasn't deployed," he says.

Mr Lefler has dual US-Israeli citizenship and says that because of his access to classified material, his commanding officers gave him an ultimatum: give up your clearance or give up your Israeli citizenship.

Mr Lefler held on to his Israeli passport and hoped for a deployment that didn't require clearance, but he never left US soil.

Now, without a combat tour, he has a lingering sense that his eight years of service - and the military's investment in training him - were wasted. He says he feels like he hasn't fully served and wonders why the military didn't utilise his skills more effectively.

"It's not like I was bloodthirsty and wanted to hurt people," Mr Lefler said. "It was more of a camaraderie feeling for me to fight alongside my brothers, travel to different countries and serve my duties as a Marine."

'Stuck in the States'

Justin Lago has a similar story. Like all Marines he considers himself a rifleman first and foremost.

But his assigned role was documenting Marine Corps operations as a combat correspondent.

I wanted to fight, not to earn a badge of honour or a ribbon. It was the job I trained for Justin Lago, Former US Marine

After the terror attacks on 11 September 2001, Mr Lago and his fellow Marines itched to be sent into battle.

"There was a lot of salty talk about what we would do once we found out who and why. A lot of misguided rage," Mr Lago says.

"Everyone just wanted to be the first on a flight to fight. It didn't matter what job we had. It always went back to who we were as riflemen first."

But time ticked on and Mr Lago's boots never touched the ground in Iraq or Afghanistan. He was deployed to the US base in Okinawa, Japan, but mostly watched the war from the US.

An expert shooter, he wished he was watching a comrade's back or saving a life rather than being "stuck in the States".

"I wanted to fight, not to earn a badge of honour or a ribbon. It was the job I trained for," Mr Lago said.

Irresistible pull

Others who have seen conflict are drawn back again and again.

Michael DeVaughn served as a military policeman in both Iraq and Afghanistan. When his service ended he decided to further his education.

Image caption For some, the lure of battle is too strong

But after three years in the army reserves, the pull of combat was too strong. He recently volunteered for a third tour, and will be deployed to Afghanistan in August.

Mr DeVaughn felt his skills were wasted in his civilian life. But mostly, he just really enjoyed his job as a military policeman, deriving an enormous feeling of accomplishment from having carried out his difficult and stressful role well - and surviving.

And, like Mr Lefler and Mr Lago, he found the unique camaraderie of the military exerts an almost irresistible pull.

"Everybody knows that at one point that your life could depend on the person next to you or vice versa," Mr DeVaughn says. "You don't feel that at a desk job."