The Marines (search) have one of the roughest assignments in Iraq: pacifying the perpetually restive Anbar province, home to Fallujah, Ramadi and Haditha, all sites of heavy American casualties since the insurgency went into high gear last year.

Underscoring the heavy load, the Marines have taken casualties disproportionate to their numbers in Iraq.

Marines number more than 23,000 out of 138,000 members of the U.S. armed forces in Iraq, or 17 percent. Yet they have lobious assault vehicle in Haditha in western Iraq. Just two days earlier, seven other Marines died.

Sen. Joseph Biden (search), ranking Democrat on the Senate Foreign Relations Committee, called the Iraq campaign now a tough situation "that's going to get tougher before it gets easier."

"The truth of the matter is that we've made some pretty significant miscalculations in term of policy form the outset, and we leave these Marines in a very, very tough spot," Biden said Thursday on CBS's The Early Show.

"There's such a gap between the reality on the ground in Iraq ... and the rhetoric back here" from the Bush administration, Biden said, asserting that the United States is in a race against time to get Iraqis trained to defend themselves, but using "too few troops in a place that needs a heck of a lot more security."

Some military experts pointed to Wednesday's attack to note the Marines are performing duties somewhat different from those for which they are organized and equipped. The amphibious vehicle, for example, was designed to get troops ashore and is less armored than some other infantry carriers.

"It's basically designed to get across the beach and get a few dozen miles inland," said John Pike (search), a military expert with Globalsecurity.org. "The point being, once (Marines) had managed to secure the beachhead and get a few miles inland, the Army would come ashore and take over from there."

Beyond that, occupation and stabilization duties often require expertise and equipment distinct from amphibious assault and the rapid capture of enemy-held territory, experts said.

"The entire Marine force was designed around the concept of amphibious warfare, which is a good deal different from the kind of conflict they're fighting in Iraq today, hundreds of miles from the sea," said Loren Thompson of the Lexington Institute think tank. "The Marines are a light force; they kick in the door but they are not supposed to occupy all the rooms."

Still, the service has tried to adapt to changing missions, studying concepts like urban warfare and nonlethal weaponry. Marines took the lead in supplying food during a famine in Somalia in the early 1990s.

They, along with the Army's 3rd Infantry Division, comprised the leading forces in the drive toward Baghdad in 2003.

Since the invasion of Iraq, the Army's effort to keep its troops fresh by rotating them in and out of the country has created a need to use the Marines as a stabilization and counterinsurgency force in parts of the country, experts said.

The Marines killed Wednesday were part of a sweep for insurgents in communities along the Euphrates River between Baghdad and the Syrian border. At the Pentagon, Brig. Gen. Carter Ham said similar operations were under way in several communities at once, to prevent insurgents from skipping to towns without a strong U.S. presence.

He suggested the attacks on the Marines were the insurgents' response to their stepped-up operations.

The Marines killed Wednesday were assigned to the 3rd Battalion, 25th Marines based in Brook Park, Ohio, a Cleveland suburb, and attached to the Regimental Combat Team 2. Nine of them were from a single smaller unit in Columbus. A civilian translator also was killed and one Marine was wounded.

Six more Marines were killed in Haditha earlier this week. A seventh was killed by a car bomb in Hit.

In November, Marines led the assault to retake Fallujah, an insurgent stronghold 40 miles west of Baghdad. They had regular clashes with insurgents there and in nearby Ramadi for months before.

In January, 30 Marines, along with a Navy sailor, were killed when their helicopter went down in bad weather. The military, however, still has not issued a finding on the cause of the crash.