Marine Sgt. David Christoff Jr. of Ohio was on his second tour when he was killed by a roadside bomb last week in Iraqi insurgent heaven – Anbar province. The next day, Marine Lance Cpl. Robert Posivio III of Minnesota died on his third Iraqi deployment.

In the same week, another Marine on his third tour in Iraq, Lance Cpl. Benito Ramirez of Texas, died from a homemade bomb blast in Anbar province.

At least four of the last 10 U.S. military personnel killed in Iraq were on their second or third combat tours.

For those wondering how the U.S. military got “overstretched and overstressed,” to borrow the recent phrasing of Rep. John Murtha, D-Pa., and a Vietnam Marine veteran, look no further than these multiple deployments.

The Iraq war is being fought on the backs of too few troops recycling back into the combat zone because of a Pentagon that refuses to acknowledge planning errors and stomach the cost of expanding the U.S. military.

Never before in an American war has a relative handful of U.S. troops been asked to take so many bites of the combat apple.

The situation is worst among Marines bearing the brunt of combat in Anbar province, and among special operations forces heavily used in both Iraq and Afghanistan.

The trend will only worsen now that many reservists and National Guard troops are hitting statutory limits on how many months they may serve overseas.

Murtha used the “overstretched and overstressed” phrase as he talked recently about an ongoing military investigation into whether some Marines murdered up to 25 Iraqi men, women and children last November in the insurgent stronghold of Haditha.

Admittedly, Haditha is bad business. It’s the same Euphrates River town where 20 Marines were killed last year.

The city is the periodic target of insurgent cleanups: Operation River Gate last fall; Operation Scimitar and Operation River Blitz before that. But it always recycles back to the same story: Haditha has no central law apart from terrorists and insurgents.

Some say this is thanks to the battle for Fallujah in 2004, that pulled away U.S. forces and left Haditha wide open to fleeing terrorists who reportedly murdered the local police.

However it happened, the effect has been lethal. Guardian reporter Omer Mahdi found DVDs at the local souk, or market, showing the execution, disfigurement and torture of local residents for apparently minor infractions of religious law.

Meanwhile, some of the Marines who served in Haditha in the past are now part of another operation – trying to rid themselves of post-traumatic stress disorder.

A recent study in the Journal of the American Medical Association said 35 percent of returning Iraq vets sought mental health services within a year of coming home – and half of those were referred for more treatment, five times the expected rate.

“Combat Diary,” the A&E documentary that aired last week about what Ohio’s Lima Company of Marine reservists faced around Haditha, left little to the imagination:

“Smiling, waving kids who may be intentional decoys, lulling Marines into a false sense of security before a massive roadside bomb hits.

“Insurgents pumped up on drugs that make them irrational and highly dangerous.

“House-to-house “clearing” operations in which the enemy is elusive and deadly.

After fighting the Iraq war on the cheap with too few troops to impose immediate order, then creating ill will and a dangerous security vacuum with anti-Baathist purges that busted up the entire Iraqi army, the Pentagon shouldn’t be surprised when its “overstretched and overstressed” troops need additional support, training and morale boosters.

Instead of being surprised, maybe it’s time for some planning.