Trying to control the 600-kilometer (370-mile) border in this desert region is a tall order for US forces, who regularly accuse Syria of doing too little to prevent insurgents crossing into Iraq.
The area is also renowned for illegal trade, with cross-border smuggling a way of life for many of the local tribes.
"They have smuggled for thousands of years and they will still do it" in the future, says Colonel Stephen W. Davis.
"The region is a general supply point for everything. They smuggle whiskey, cigarettes, peanuts. It's a tribal business," he adds.
But the smuggling routes are also open to foreign insurgents, mainly from other Arab countries, albeit crossing in small numbers, he says.
"They do not bring battalions, they bring the leadership, the financial man, the demolition expert," says Davis.
"We have intercepted mass ammunition supplies" and even "anti-aircraft weapons have come through the border," he says.
In an effort to police the lawless zone, long rebellious to central government, US forces are trying to improve ties with the local Sunni tribes.
"This area never liked government control, so we and they (Iraqi authorities) are building alliances. Business rules here," says Davis.
"We are under no illusion that they love us, but they hate the foreign fighters, and some welcome US forces for their own survival," he adds. US authorities have repeatedly accused Syrian authorities of allowing rebels, including those associated with Abu Musab al-Zarqawi's Al-Qaeda group in Iraq, to use their territory as a staging post and a safe haven.
In Huseybah, a rebel stronghold on the border, US helicopters have just spotted men aboard two four-wheel-drive vehicles unloading weapons.
From a border post, which has been turned into a bunker, US snipers resting on sand bags open fire. Spotters shout out co-ordinates as marines rush to man mortar tubes. Orders to fire crackle over the radio and some 10 mortar shells slam into a house several hundred yards away. A man runs across a street into a mosque, pulling up his white robe in an effort to sprint.
"There are only bad guys, no good guys in this town," says Corporal Michael Nguyen.
"We are not interested in taking prisoners. If you are here to fight us we will kill you," adds Davis.
Meanwhile, a marine lying on a sandbag keeps an eye on a woman, just across the border, who is hanging up her washing next to a giant poster showing the late Syrian president Hafez al-Assad, and a signpost in both English and Arabic which reads "Welcome to Syria."
US forces said they bombed three buildings in the town on Saturday, killing an estimated 10 suspected rebels.
The air strikes took place after US forces on the ground came under fire while conducting simultaneous raids on two rebel houses in different parts of the town, the military said. A third air strike was later carried out against another suspected militant house with fortified fighting positions.