With Theresa May - preparing to enact Article 50 officially starting the Brexit process from the EU - set to meet Trump tomorrow as the new US president's first meeting with an international leader to lay the groundwork for a U.S.-U.K. trade deal, the outcome will provide the first test for how world leaders can deal with Donald Trump, who has put the world on edge with his recent push for isolationism and trade protectionism, culminating most recently with his tweet that Mexico's president needn't bother visiting if Mexico will not pay for the wall along the Mexican-US border.

“As we rediscover our confidence together –- as you renew your nation just as we renew ours –- we have the opportunity, indeed the responsibility, to renew the special relationship for this new age,” the U.K. prime minister will tell Republican lawmakers gathered in Philadelphia on Thursday, according to excerpts from her prepared remarks. “We have the opportunity to lead, together, again.”

As Bloomberg notes, the good news for May, who’s due to meet Trump at the White House on Friday, is that he’s eager to cement relations and nail down a U.K. trade deal too -- for his own reasons. He’d like to further drive a wedge into a fractured Europe and strengthen at least one trade relationship as he exits the Trans-Pacific Partnership and prepares to renegotiate Nafta.

A close relationship between the U.S. and U.K. would prove that neither nation is turning inward -- Trump after an election victory fueled by his “America First” campaign, and May as she takes Britain out of the European Union after last year’s Brexit referendum. So May is opting to brush aside the worldwide protests that followed Trump’s inauguration and worked hard to secure Trump’s first meeting in office with a foreign leader.

For Trump, who will also speak Thursday to the Republican lawmakers in Philadelphia, it’s a chance to show that world leaders are eager to meet with him despite the protectionist and unilateralist themes of his inauguration address. He pledged then that “every decision on trade, on taxes, on immigration, on foreign affairs will be made to benefit American workers and American families.”

But while May will offer reassuring words in her speech to the gathering of lawmakers, she will implicitly reject the idea of a U.S. withdrawal from international entanglements. “The leadership provided by our two countries through the special relationship has done more than win wars and overcome adversity,” she’s planning to say. “It made the modern world. The institutions upon which that world relies were so often conceived or inspired by our two nations working together.”

Success for May’s visit would be a clear commitment from Trump to a bilateral trade deal -- even though the actual deal will be much harder to nail down and wouldn’t take effect until the U.K. completes its extrication from the EU some years from now. But it’s important politically that she isn’t seen as compromising British interests. “They want completely different sorts of bilateral trade deals,” Colin Talbot, professor of government at Manchester University, said in a phone interview. “There’s a danger Britain gets stuffed. A quick trade deal is generally a bad trade deal.” May was clear on Wednesday that she won’t follow Trump wherever he goes. “I am not afraid to speak frankly to a president,” she told Parliament. An aide spoke of aiming for a grown-up relationship.

However, a successful meeting would also put the European Union in a weaker position, as it would show that countries potentially contemplating leaving the union, have a green light in contemplating bilateral deals with the US.

As a result, moments ago, French Finance Minister Michel Sapin said “Madame May can go see whoever she wants. I understand she goes to see the new U.S. president given the history between the U.S. and the U.K" However, he added, "she is not going there to negotiate," Sapin says in interview in Brussels. "Neither she nor Mister Trump are in a position to negotiate. It’s a courtesy visit."

Both May and Trump are looking forward to proving him wrong.

Sapin also opined on the topic of U.K. lowering taxes, saying that “lowering taxes alone is weakening yourself. It doesn’t scare others."

Judging by the concerned tone in Sapin's voice, it appears to be scaring at least one country.