The images that streamed from inside Afghanistan after the fall of Kabul were a propaganda dream for the United States government. Afghan women, after years of cruel subjugation by the Taliban, were daring to shed their veils and to expose their faces once again to the world and to sunlight. It was America – with a little help from Britain – that had made this happen. The smiles shone brightly; eyes shimmered with joy.

Cut now to the Prince Sultan US Air Force Base in Saudi Arabia. An American fighter pilot is climbing into a Chevrolet Suburban to make a trip off-base. The pilot, a Lieutenant Colonel, is not wearing Air Force uniform. This officer of the US military is instead wearing something called an "abaya". A slightly less restrictive version of the burqa, it is a suffocating, head-to-toe robe that allows no glimpse of flesh and has only two slits for the eyes.

If this seems a little rum to you, you are not alone. The officer in the Suburban has a name, and the brass in the Pentagon are getting a little tired of hearing it. It belongs, of course, to a woman. Not any woman, but the first female to become a fighter pilot in the US Air Force since the military first decreed that women could serve in combat in 1993. Today, she is also America's most senior female officer.

But the career of Martha McSally may be about to be shot down. She and other women serving in Saudi Arabia wear the abaya robes when they leave base because they are obliged to. US military regulations insist that servicewomen must always wear an abaya when leaving the base. There is more. Out of respect for local religious custom, Lt Col McSally may not drive the car herself. (That is almost funny. She can pilot a plane, but is not permitted to take the wheel of the Suburban.) In fact, she is not even allowed in the front. And she must have a male escort.

McSally, 35, is a veteran pilot of the A-10 Warthog – a single-seat aircraft with a powerful gun that is usually deployed to seek out and destroy tanks. She has patrolled the hazardous no-fly zones in northern and southern Iraq. Most recently, she been given control of all search-and-rescue missions in Iraq, and now also in Afghanistan. Hers has been a dream career, marred only by her deep-seated distaste for the abaya regulation. She first heard of it in 1995, long before she was herself sent to Saudi Arabia, and has been fighting ever since to have it rescinded. Apparently, however, her commanders have not been listening. And so now she is taking the route of litigation and suing the Pentagon for violating her constitutional rights. More specifically, she is suing the man in charge, the US Defense Secretary, Donald Rumsfeld.

The timing of the suit is either way off or it is perfect. Mr Rumsfeld has a war to run. Moreover, this surely is not the best time to disturb an already thoroughly complicated relationship with Saudi Arabia. On the other hand, how can the US continue to require its own servicewomen to cover themselves in the Kingdom, when it is celebrating its success in liberating women from the custom in Afghanistan?

It is not hard to see how the regulation came about and why it has endured. To those behind desks in Washington – mostly men – it must seem like an almost cost-free gesture to a country that may be an ally of the West, but that remains a fundamentalist Islamic society. It is, indeed, the homeland of Osama bin Laden. Keeping the lid on Muslim sentiment that would chase the Americans off the sacred Islamic soil of the Kingdom remains one of the trickiest challenges for the monarchy and the US. If obliging female servicewomen – and there are about 1,000 of them currently affected by the regulation – to tent up every time they go off-base helps demonstrate respect for Islamic traditions, so much the better. And the old adage always holds good: when in Rome, do as the Romans do.

Equally easy to grasp, however, is McSally's viewpoint. What is curious is the support she is getting in the US. It is coming partly from feminist groups with left-wing agendas: the National Coalition of Women's Organisations is among those to have lobbied the Defense Secretary. But at the same time, McSally is being lauded by conservative politicians and lobby groups who hold the US Constitution above all else. "What makes this particularly bizarre is that we are waging a war in Afghanistan to remove those abayas, and the very soldiers who are conducting that war have to cover up," Bob Smith, a Republican senator from New Hampshire and a noted conservative, told The Washington Post.

Similarly offended is Lionel van Deerlin, a former US Congressman from San Diego, California. "What mockery that the reforms in Afghanistan should be achieved while, in friendlier surroundings, we abjectly copy the medieval mode of Saudi Arabia," he noted.

Most startlingly, the McSally case has been taken up by none other than the Rutherford Institute, a notorious right-wing advocacy group that came to the fore when it took Paula Jones under its wing and pressed her sexual harassment suit against former president, Bill Clinton. The institute has taken it upon itself to file the suit against Mr Rumsfeld on her behalf. It runs to 14 pages and requests no financial compensation, but asks simply that the military desists from humiliating servicewomen in Saudi Arabia. At its core is McSally's objection to what she recently called the US treating her "like a piece of Muslim property".

It does not help that American policy appears to be hopelessly inconsistent. The US State Department, for example, has no abaya requirement for its personnel at the US embassy in Saudi Arabia. It is hard to imagine a female chargé d'affaires or ambassador posted to the kingdom going about her business in Riyadh covered from head to toe. Nor does the cover-up regulation apply to the wives of male military officers. Servicemen posted there, moreover, are expressly forbidden from donning traditional Muslim clothes. Thus, when a female officer leaves the US force in her abaya, she will be escorted by burly American GIs with crewcuts. "We hardly blend in," McSally quipped recently.

Nor is McSally impressed by the notion that America should make cultural allowances for whatever country it stations its soldiers in. "If it were in our national security to deploy to South Africa under apartheid, would we have found it acceptable or customary to segregate African American soldiers from other American soldiers and say, 'It's just a cultural thing'?" she asked, addressing a group of girl students at the National Cathedral School in Washington recently. "I don't think so. I would hope not."

In the end, she says, the American military has to pay attention to the US Constitution first, and to the values that America holds dear – not to the local customs and religious dictates of whatever state is acting as host to an American base. "When those customs and values conflict with ones that our Constitution is based on, and that women and men in uniform died for in the past, that is where you draw the line."

Forcing the women to wear the abayas has practical consequences, too, she argues. The effect is to demean her and her colleagues and that, in turn, undermines their positions on base. When she climbs, virtually incognito, into the back of the Suburban, she is under the escort of officers who are junior to her. It has taken many years to encourage equal respect for men and women in the military culture. But in Saudi Arabia, the abaya rule is helping to undo all that has been achieved.

In response, the Pentagon has replied simply that servicewomen must abide by the regulation when stationed in Saudi Arabia, because Americans are there as guests of that country. Those are carefully chosen words. You could argue that Americans are rather more than guests – they are defenders of the Kingdom. You could also highlight the shortcomings in the relationship. Saudi Arabia has come under harsh criticism for failing to co-operate fully with Washington in crushing terrorist cells worldwide. It might also be noted that at least eight of the 11 September hijackers in the US hailed from the Kingdom. And yet America fears offending its rulers. It needs its base there. And the West needs its oil.

Lt Col McSally, who was given her wings even though, at 5ft 3ins, she is theoretically too short to fly for the US Air Force, insists that she is not acting out of some feminist fervour. "The last thing I ever wanted to do was make a big deal out of being a woman," she told The Washington Post. But she is clear in her head that the package of regulations imposed on her and her female peers in Saudi Arabia "abandons our American values that we all raised our right hand to die for". Is she risking all that she has achieved in her career – including being the first American women to fly in combat – because of her commitment to this one issue? "I don't know," she said. "I try not to think about it."

What happens next? The Pentagon has conceded that the issue is now "on people's radar". It is not quite clear what they mean by that, however. The clear hope of the Lieutenant Colonel is that the brass will decide to settle with her by quietly ditching the offending regulation – with who knows what effect on Saudi public opinion. Otherwise, several months from now, she will find herself fighting her cause in court. And even a fearless pilot such as McSally might tremble at facing a foe as formidable as Rumsfeld.