In the world according to President Donald Trump, the United States and Britain are about to make their special relationship great again. Having cast off decades of global niceties, the United States by putting him in the White House, Britain by voting to leave the European Union they can now forge a magnificent future.

Trump is readying proof. As he prepares to host Prime Minister Theresa May of Britain at the White House, he intends to begin discussions on a free-trade agreement that would deepen commercial ties between their two countries.

A trade agreement would make for useful political theatre on both sides of the Atlantic, particularly as President Barack Obama had pointedly warned that a deal would not easily happen. But as a spur to economic expansion and hiring, a deal would almost certainly amount to very little.

Until Britain actually leaves Europe, a step most likely at least two years away, it is legally barred from entering a trade agreement with any country. Were a deal quietly negotiated to take effect after Britain's departure, its economic punch would probably be modest. Tariffs between the two countries are already minimal, meaning there are relatively few trade impediments left to clear away.

The areas of business that could be opened to increased competition tend to be the most politically sensitive: agriculture, government procurement and defense. Legions of lobbyists would mount ferocious battles to preserve the privileges of now-favored industries.

U.S. federal government contracts would attract keen interest from British defense and aerospace companies, especially as Trump has vowed to rebuild the military. But opening such business to foreign competitors in a trade agreement would directly collide with Trump's "America First" mantra.

"If President Trump genuinely means 'buy American, hire American' above all else, then there's going to be very little left to offer other countries," said Adam S. Posen, a former member of the rate-setting committee at the Bank of England and now president of the Peterson Institute for International Economics in Washington. "There is no way in hell this is a massive trade deal. It would take massive effort to create a trade deal of even minimal effect."

Trade, it is worth noting, tends to involve trade-offs. Negotiations are politically fraught, because they expose domestic industries to the prospect of greater competition with overseas companies. In exchange, domestic consumers gain access to more and cheaper products.

British steak aficionados may cheer the shrinking of steep tariffs that limit the influx of U.S. beef. The British beef industry would probably respond less enthusiastically. It would seek protection, particularly as Britain's exit from the bloc would eliminate European subsidies for British agriculture. Any proposed opening of agriculture would most likely provoke British opposition to genetically modified crops.

British insurance companies may be eager for a greater crack at the enormous U.S. market, but that would be outside the scope of a potential deal. In the United States, the insurance industry is regulated at the state level, meaning the Trump administration would lack the authority to engineer an opening.

The biggest impediment to a substantial deal between Britain and the United States is the similarity of the two economies, and especially the importance of financial services on both shores.

Opening finance to greater competition would require that British bankers pledge fealty to U.S. regulations. Either that, or the rules would have to be opened and renegotiated by both sides. That might happen in the same way that Trump might perhaps renounce shiny ties and golf in favor of tie-dye and Zen meditation.

For U.S. companies, the appeal of giving something up to gain access to the British market has been diminished by the very thing that makes such a trade permissible: Britain's looming exit from Europe, known as Brexit.

Britain's inclusion in the European Union and its enormous single market has made Britain a prized center for manufacturing and corporate headquarters for multinational companies. U.S. investment banks have been able to set up shop in London and use these bases to serve clients from Ireland to Romania, as if operating in one country.

But once Britain leaves Europe, these benefits would almost certainly disappear, raising endless questions about the future terms of trade across the English Channel.

Already, major banks are publicly outlining plans to shift some operations to cities within the European Union, exploring Dublin, Paris and Luxembourg. Already, major multinational companies are expressing reservations about sinking the next investment in Britain.

All of this makes a trade deal less compelling, diminishing the stakes, leaving politics as the most meaningful context.

Last year, before Britain's referendum, Obama visited the country and urged voters to opt for remaining in Europe. He sought to undercut a key aspiration of those urging a break from the bloc, the prospect that a more independent Britain could strike a free-trade deal with the United States.

Washington had little interest in striking deals with single countries, Obama said, preferring to focus on giant multilateral trade arrangements like the sprawling Trans-Pacific Partnership (a deal Trump just renounced). If it sought a trade deal with the United States, Britain would land "in the back of the queue," Obama warned.

With Trump now in the White House, Britain's place in the queue has effectively been upgraded, a handy development for May as she presses ahead with Brexit. She is under pressure to demonstrate that Britain, once liberated from Europe, can harvest bounty via trade with faster-growing, more innovative parts of the globe. A trade deal with the largest economy on earth would presumably check that box.

"It is clearly in the British government's political interest to show that it can negotiate a free-trade agreement with the United States," said Stephen Woolcock, a trade expert at the London School of Economics. "The question is what the economic costs and benefits of such an agreement would be? These would seem to be less clear cut."

Trump could post a deal with Britain on Twitter as proof he is not, despite popular perceptions, a nativist protectionist leading the United States on a retreat from global concerns.

In a similar vein, he could use a trade deal with Britain, even a narrow, economically inconsequential one, to claim vindication for his encouragement of leaving the bloc. 

"Trump can play to his constituents," Posen of the Peterson Institute said, people "who believe that trade deals among white rich people are particularly desirable."