At around in the morning, just south of the town of Balad Ruz, a small detachment of American soldiers dismounted from their armoured vehicles and set off on patrol.
It was a quiet night, and there was little to see or hear, apart from the sound of boots crunching along the dirt tracks and the barking of the stray dogs that accompanied them.
Balad Ruz lies near the Iranian frontier.
Until a year ago, the town and its surrounding countryside was a lawless region, a haven for al-Qaeda fighters and a hideout for weapons smugglers.
Since then, al-Qaeda forces have been largely pushed out, by joint US and Iraqi military operations.
But the smugglers are still coming through, bringing in rockets, explosives, bomb-making equipment and expertise across the border from Iran.
"They could be moving stuff to go down to either SadrCity, Baquba or Baghdad," said Lt Luke Brown, scanning the countryside through his night-vision goggles.
The American Stryker armoured vehicles are fitted with sophisticated infrared and thermal imaging technology, not at the disposal of the Iraqi police and army.
But despite their hi-tech equipment, so far the Americans are having little luck.
"We do know that a lot of the arms, ammunitions and explosives that we find here, being used against the Iraqi security forces, our forces and against the Iraqi people clearly have originated from Iran," says Col David Funk.
Col Funk is the officer in charge of US forces in Diyala province, an area of eastern Iraq that shares a long border with Iran.
The Americans believe that their presence there, and their training of the Iraqi border force, is at least partly responsible for a drop in weapons smuggling.
But rockets and other ordnance are still coming through, and the US military believes that some of it is being supplied by the al-Quds force, a branch of the Iranian Revolutionary Guard.
"There are those in the Iranian government who clearly do not want Iraq to become a strong nation," says Col Funk.
"It suits Iran if Iraq is a sort of puppet neighbour as opposed to the very strong nation that it has the potential of becoming. It suits their needs because it keeps a weak neighbour on their western flank."
With less than a week to go until the Parliamentary vote, the streets of Baghdad and other Iraqi cities have become crowded with election posters.
A profusion of faces stared down at the traffic.
The Americans have accused Iran of trying to influence the outcome of the poll - masterminding a controversial blacklist of candidates seen as targeting Sunni politicians unsympathetic to Tehran.
Maj Gen Steve Lanza, the chief spokesman for US forces in Iraq, says that Iran is now moving from what he calls "direct influence" in Iraq to using "indirect" means.
"What we've seen in some cases is what we call a malign influence. And that influence in some cases has come from Iran, by money and by resources that are being applied within the state, money that is coming in to influence and shape the elections."
In the past few weeks, accusations of foreign funding have flown back and forth between the various political parties.
In the absence of a legal paper-trail, the allegations are hard to prove. In any case, such financial contributions would not be illegal.
Iraq's laws covering the financing of political parties are vague.
But Western diplomats in Baghdad believe that millions of dollars of Iranian money have been flowing into the country to fund Shiite parties.
The history of Iraq's relations with Iran is a patchwork of close cultural links punctuated by tension and mistrust.
As a member of Iraq's Sunni minority, Saddam Hussein feared the power of the Shia Muslims.
In the 1980s and 1990s, during and after the decade-long war between the two countries, many Shia leaders fled Iraq and spent their exile in neighbouring Iran - a theocracy governed by Shia clerics.
But the toppling of Saddam Hussein has led to a Shia renaissance.
Many of those Shia politicians have now returned from Iran to positions of power in the new Iraq.
Movement and links between the two countries, once virtually impossible, are now blossoming.
Nowhere is this new relationship more apparent than in the city of Najaf, home to the shrine of the Imam Ali.
The city is a magnet for Shia pilgrims.
They flock here from across the Islamic world, arriving in their millions every year.
The biggest groups come from Iran.
On a recent Friday, several hundred mostly Iranian pilgrims were kneeling on the carpeted floor in the courtyard of the shrine, chanting in Persian, beating their chests.
Released from its Saddam-era constraints, Najaf is rapidly regaining its place as the epicentre of Shia religious and political power.
Sheikh Mohammed al-Yakoubi is one of the top religious authorities at the shrine.
He says that unlike the clerics of Iran, he and his fellow Ayatollahs at Najaf try to remain detached from politics.
They have resisted considerable pressure to publicly endorse any Shiite parties.
But, in carefully-worded language, he admits that there are those who would like to harness that Shia power for their own ends.
"Certainly there are schemes to manipulate the outcome of the elections in a way that would suit the major regional players."
Saudi Arabia, Syria and Turkey all have an interest in what happens inside Iraq, as of course does the United States.
But many believe it is Iran whose influence is now felt strongest.
Iyad Jamal Aldin is a Shia cleric and a candidate in the upcoming election.
He is also a fierce critic of Tehran's involvement in Iraq.
At the end of last year, Iranian forces seized an Iraqi oil field near the Iranian border, an area that has never been properly defined since the Iran-Iraq war.
The dispute was eventually resolved peacefully: Iran's flag was lowered and its forces withdrew.
But Aldin cites the incident as one of many reasons why Iraq should be wary of its neighbour.
And, he says, the wholesale dismantling of the Iraqi army by the United States following the invasion of 2003 is partly to blame.
"Americans opened the door for Iranians to occupy the country," he said during a recent meeting at his home in central Baghdad.
"And now they want to withdraw. The Americans have to stay until we have a proper army."
But the United States is not staying.
'Hard to soft'
It plans to withdraw all its combat forces from Iraq by the end of August this year, in preparation for a near-complete military departure by 2012.
Back in Diyala province, within sight of the border with Iran, another platoon of American soldiers is on patrol.
The commanding officer, Capt Andrew Marsh, calls their activities here "on the job training" for the Iraqi border force.
He is confident that the Iraqi military will be capable of securing the border on their own, once the Americans leave.
The US military presence here on the border is increasingly moving from what they call "hard" to "soft power".
While out on patrol, Capt Marsh and his men took time out to inspect flood damage after recent heavy rains.
They will rebuild bridges and roads that have been swept away.
But as they reduce their numbers, that "soft power" is also getting softer.
On either side of the border everyone is conscious of the fact that the Americans will not be here for much longer.
The question is, what happens once they are gone. Because Iran, by virtue of geography if nothing else, is here to stay.