Negotiating with the U.S. was always going to be difficult. In addition to being the world’s largest economy (Britain is fifth largest), the U.S. boasts a population five times that of Britain. Only two powers come close to negotiating on parity with Washington: the EU and, especially in recent years, China. As an EU member, Britain wields the influence of a 28-member trading bloc, which has a combined economy that rivals the United States. On its own, the U.K. is decidedly less powerful.

“Access to the U.S. market is, blow for blow, worth more than access to almost any other market on Earth,” Dmitry Grozoubinski, a former Australian diplomat and trade negotiator at the World Trade Organization, told me. “From that perspective, whenever you’re talking to the U.S., the deal almost always means more to you than it does to them.”

This doesn’t mean that the U.S. lacks the will to strike an agreement with Britain. After all, Trump has repeatedly expressed his enthusiasm for a free-trade accord, even going as far as to advocate a no-deal Brexit—in which the country would leave the EU without an agreement outlining the terms of its withdrawal—just to see one. (Britain can’t strike its own trading policy until after it has left the EU.) His enthusiasm was echoed in a letter of support to Johnson last week, in which a group of 45 U.S. senators, out of 100, pledged to back a bilateral trade agreement regardless of how Britain leaves the EU. (In the U.S., all trade deals require congressional approval.) Secretary of State Mike Pompeo reaffirmed this week that the U.S. will be ready “pen in hand … to sign a new free trade agreement at the earliest possible time.”

Read: Inside Donald Trump and Boris Johnson’s special relationship

Still, the viability of a trade deal will largely depend on what concessions London is willing to make—and Washington is expecting many. Among the negotiating objectives set out by the U.S. trade representative in February is the insistence that Britain “remove expeditiously unwarranted barriers that block the export of U.S. food and agriculture products,” such as chlorine-washed chicken and hormone-treated beef. (Both products are banned in Britain for health and safety reasons; The U.S. argues that such concerns are overblown.)

Agriculture isn’t the only potential stumbling block: Privacy and data flows (the U.S. has a more liberal approach to data privacy than Britain, which operates under the EU’s more stringent GDPR standard); digital taxes (the U.S. opposes a proposed measure that would see a 2 percent tax on American tech firms’ U.K. revenue); and drug pricing (the Trump administration has long been critical of how Britain’s National Health Service assesses the price of medicines) are all areas of disagreement.

There’s also the issue of China. In the trade deal agreed with Mexico and Canada last year, Washington included a clause stipulating that if any of the parties signed a free-trade deal with a country that has a nonmarket economy, in which the government plays a significant role (read: China), the other signatories can take action and even terminate the agreement. Britain’s willingness to work with China—particularly on issues such as Huawei, the Chinese tech firm to which Britain had granted some access to parts of its 5G network before reversing itself following U.S. criticism that the company could allow China to spy on sensitive communications and data—is one of several foreign-policy areas where the two countries diverge. It’s not inconceivable that the U.S. would demand the same restrictions on Britain that it did with Canada and Mexico. Its negotiating objectives already suggest that it will.