The image is deliberately provocative and arresting. It depicts the White House with the gaps between its pillars draped in swastika flags while a battalion of German troops in coal-shuttle helmets marches past.

The concocted photo dominates the page of the website of Britain's influential Conservative Voice, an organization whose ranks includes many conservative members of the British and European parliaments and couple of co-chairs of the Conservative Party itself.

Under the headline "USA Threatens Britain Not To Leave EU" the article begins: "The Obama administration has warned British officials that if the U.K. leaves Europe it will exclude itself from a U.S.-EU trade and investment partnership potentially worth hundreds of billions of pounds a year, and that it was very unlikely that Washington would make a separate deal with Britain."

This is true, widely reported and it makes admirable sense. It was a British prime minister, Lord Palmerston in the 1850s, who observed that "Nations do not have permanent friends or permanent enemies; only its interests are eternal."

It is in the American interest to keep Britain inside the European Union, as a reliable ally of long standing which speaks the same language and shares much of the same culture, values and beliefs, from free speech to free markets and free trade.

A British presence is the best guarantee that the European Union won't succumb to France's statist and protectionist instincts nor to German mercantilism.

(It ought to mean a British counterweight to those neutralist and pacifist tendencies that is making Europe less of a useful ally but Britain's own defense cuts are steadily eroding the country's military capabilities.)

Still, Britain remains the most congenial and reliable of allies and a Europe without it will be a far more complex beast for U.S. policymakers to work with. And there remains sufficient Anglophile sentiment in Congress for the proposed free trade agreement with Europe to be a much easier sell if Britain is in than if Britain is out.

The logic is therefore inescapable. In its own interests, the United States should and will deploy as much influence as it can in London to ensure that the lemming-like drift of the Conservative Party out of Europe should be arrested and reversed.

They also firmly say that it is in the British national interest to remain inside Europe. U.S. officials have been saying so, in public and in private, and are considerably frustrated that their friendly advice has had so little impact.

That is why the friendly advice from Washington has escalated into not-quite-so-friendly warnings.

The evidence of that offensive Photoshopped image of the White House in swastikas suggests that many British conservatives aren't only refusing to listen but that they take this advice as some form of bullying diktat. It is the equivalent of those deliberately insulting images in Spanish and Greek magazine of German Chancellor Angela Merkel sporting a Hitler moustache and wearing a swastika armband.

The Conservatives don't learn. They have already lost two prime ministers -- Margaret Thatcher and John Major -- through the party's internecine wars over Europe. They could now lose a third, David Cameron.

And they could lose much more than a mere politician. A Britain outside of Europe, which take half its trade and which guarantees the country its $1.1 trillion foreign investment and which employs 15 percent of the workforce, might not plunge into poverty. But its chances of future growth would be very much diminished. Foreign investors would be markedly less attracted to a country that doesn't have a free trade deal with Europe, the world's biggest market.

Moreover, given U.S. irritation at such a British exit from Europe, there would be little support from across the Atlantic. Those conservative voices urging exit speak fancifully of a Britain liberated from its European shackles to trade with China, India and the emergent markets. This ignores the reality that Britain today sells more to Ireland than to China and India combined. And those European shackles haven't prevented Germany from exporting more to China than all the other EU member states combined.

Possibly in response to some hints from Washington, some of the other EU countries, notably Germany, Holland, Denmark, Sweden and Poland, have started discreet internal discussions on ways they might help to save the British from themselves, or rather from their viscerally anti-European conservatives.

The image of the White House as Hitler's bunker says everything one needs to know about the mentality, not to mention the manners, of these nationalist Neanderthals for whom it is always 1940 when Britain stood alone against the Nazi hordes. Sadly, they seem even to have forgotten which allies came to the rescue.