Britain is the only Western country that hasn't banned the stimulant. They should do so now, as sales likely fund al-Qaeda.

But whereas using or importing khat is illegal in the U.S. and most European countries, it is completely legal in the United Kingdom, where some 250,000 Somali immigrants have taken up residence. Nothing in British law stands in the way of at least 30 tons of freshly cut Yemen khat arriving at Heathrow Airport every month. By allowing this huge khat loophole to persist, our  ally in the global war on terror has become a forward staging ground for smuggling to the rest of the world, most notably to Somali immigrant communities in many American cities. Millions (maybe even billions) of British pounds and American greenbacks are flowing back to Yemen, Somalia, and other terrorist-harboring states.

I know all of this because I first investigated the international khat trade almost ten years ago while working as a reporter in Dallas, Texas. I’d noticed that local airport customs inspectors at Dallas/Fort Worth International Airport were turning up loads of the stuff on young British citizens, “mules” arriving on flights from the UK carrying luggage stuffed with weird plants most cops didn’t recognize. The plants were headed for local immigrant communities still arriving from Somalia and Ethiopia. I was surprised, for instance, to learn that airport cab drivers were chewing khat to stay on Dallas roads longer. I had no good reason to consider who might be profiting on the back end of the supply chain — not until after 9/11.

In 2005, I traveled to London to further investigate the trade in the context of the post-9/11 war on terror. I easily found the stuff for sale at any north London grocery store. Wrapped by twine in banana leaves, bundles of khat were offered for about five bucks next to bottles of Snapple, Coca-Cola, and orange juice.

British parliamentarians and political experts explained the lack of interest in following other European nations in criminalizing khat. The drug is considered so integral to immigrant culture that any measure to ban it might be construed by a restive Muslim immigrant population as an attack on culture and heritage. I also learned that British customs inspectors had never been ordered to scan for outgoing British khat smugglers boarding international flights. A khat exporter’s dream.

In fact, no one at all, save for a few counterterrorism specialists, seemed particularly concerned that the extensive transportation and distribution network enabled by the British was no doubt filling the coffers of terrorist organizations in the Horn of Africa. To be fair, I’ve seen no definitive proof that terrorist organizations are profiting from the khat trade. But if it’s indisputable that our Taliban and al-Qaeda enemies in Afghanistan happily fund their extremist activity from poppy taxation and heroin smuggling, then the only responsible assumption is that the same enemy is tapping into the khat trade in Yemen.

Changed circumstances since my reporting about khat so long ago argue strongly for British authorities to finally reverse the khat policy — and for the Obama administration to make sure that our European ally gets over its reticence to offend immigrants.

For starters, Yemen has moved front and center to the American target list as a failing state shaping up to present as much of a direct threat to the U.S. homeland as Afghanistan did until October 2001. Our English friends shouldn’t be feeling any less immune from khat-supported attacks that might come care of Yemen-based plotters.

The ranks of al-Qaeda affiliates operating in sympathetic, lawless regions of Yemen, Somalia, and Eritrea have dramatically expanded as American predator drone attacks practically wrecked the previously tranquil terrorist lifestyle in the tribal areas of northern Pakistan. Until the December 24 air raids, they had it made in Yemen. The country’s wobbly regime felt it risky to attack good fundamentalist Muslims on its own territory. But then American intelligence proved to Yemen’s regime that al-Qaeda was planning to hit it next. The Yemen government finally acted militarily. Now there’s no going back.

In retaliation for those December 24 raids, the terrorist organization proudly claimed it had deployed the young Nigerian terrorist who came so close to blowing up a Detroit-bound jetliner on Christmas Day. A calculating Obama administration wisely appears to be gearing up to hit those who sent him. Obama’s generals also are moving special operations forces to more permanent digs in Yemen to help with intelligence and counterterrorism operations.

Another circumstance that argues for the British to help starve out those who control Yemen’s khat trade is its extraordinary growth in recent years. While al-Qaeda was expanding its operations in Yemen, acreages under khat cultivation spread to historic dimensions. Whatever the reasons for its thirsty spread, khat farming in Yemen has become so ubiquitous that a nationwide water shortage was attributed to it in 2009. Khat is now considered Yemen’s biggest cash crop and one of its top exports. The drug keeps turning up with disturbing regularity in middle America, as regular busts in places like Minnesota and Pennsylvania attest.

One easy way to do damage to a khat trade that may well fund future young men who would blow up airliners over American cities is for Great Britain to simply criminalize the stuff. The beauty of this simple solution is that interdicting khat doesn’t require the fruitless sacrifices entailed in interdicting Afghan heroin, where one blocked supply route is easily replaced by an unobstructed one, or where expensive militarized eradication programs need to be tried and retired in failure.

Khat’s narcotic effect happens to be extremely time-sensitive. It works best a few days after harvest. When leaves dry out, the effect diminishes along with the street price. That’s why all the khat I saw for sale in London was always kept tightly wrapped in banana leaves and refrigerated. It had to be thrown out, shopkeepers told me, if it didn’t sell in a day or two.

That means shutting down the huge Heathrow Airport portal would so lengthen any other potential distribution line as to quite literally dry up a huge swath of the international khat business overnight, or at the very least the 30 tons a month that now move quickly from the fields of Yemen into the UK. It needs to be done because the lifeblood of international terrorists often enough is illicit cash from international smuggling enterprises.

Our stalwart allies across the pond need to find the political will to shut this spigot off. The Obama administration needs to help them find that will in a hurry, because until such time as our friends in war do the right thing on khat, any new enhanced American military front opened in Yemen is going to be less effective.