At a rally in Montana this July, U.S. President Donald Trump went on what’s become a typical rant against Europe. Referring to NATO, he declared that Americans are “the schmucks paying for the whole thing.” As with Trump’s previous such outbursts, the most striking aspect wasn’t the substance of his complaint—U.S. presidents have long urged Europe to boost defense spending—but rather his tone of open hostility.

European leaders, in their own typical fashion, met Trump’s unprecedented anger with reassuring bromides: German Chancellor Angela Merkel promised she was ready to hold “controversial” discussions, and NATO Secretary-General Jen Stoltenberg offered abstract appeals to NATO members being “stronger together than apart.” The one standout was, perhaps, the least likely. Donald Tusk, the mild-mannered president of the European Council—which sets the European Union’s agenda by convening and forging consensus among the bloc’s national leaders—reached far beyond the traditional diplomatic toolkit.

At the signing of an EU-NATO joint declaration in Brussels five days after the Montana rally, the 61-year-old Tusk pushed back firmly against Trump’s disparaging comments: “Today, Europeans spend on defense many times more than Russia and as much as China.” Tusk added a backhanded insult: “Dear America, appreciate your allies, after all you don’t have that many.” Then he offered Trump some unsolicited advice prior to his scheduled meeting with Russian President Vladimir Putin. “It is always worth knowing who is your strategic friend and who is your strategic problem,” he said.

Tusk, who served as Polish prime minister from 2007 to 2014, has emerged as a lonely voice of strident resistance against far-right attacks on European liberal democracy. Those attacks have come from foreign spoilers, such as Trump and Putin, who treat the EU as a strategic threat. But they have also been mounted by populist governments within Europe. Leaders in Hungary, Italy, and Poland have acknowledged their desire to practice forms of “illiberal democracy,” in the words of Hungarian Prime Minister Viktor Orban. The Polish government, which was alone in opposing Tusk’s re-election as European Council president, has directly implicated him in its occasionally outlandish attacks on the political establishment. Government spokesman Rafal Bochenek even went so far as to label him a “co-author” of Brexit because he had done “nothing” to stop the British referendum.

The continent’s liberal leaders have mostly responded to these provocations using liberal means: invitations to dialogue, evocations of legal procedure, appeals to reason. Tusk has not rejected those tools. But he has also led a rhetorical counterattack on Europe’s behalf—to bluntly defend the continent’s political principles and treat its opponents, whether in Washington or Warsaw, as adversaries deserving of vitriol. If the enemies of European liberalism want a fight, Tusk has made it clear that he is happy to oblige. And his ambitions clearly extend beyond winning the current round of sparring against Trump.

Tusk might seem an unlikely figure to have assumed this role. He was born in the northern Polish coastal city of Gdansk in 1957 to working-class parents—his father was a carpenter—in then-communist Poland. After studying history at the University of Gdansk, he was involved in student associations supporting the pro-democratic Solidarity trade union led by Lech Walesa. But he was never a standout figure in the pro-democracy movement. After communism collapsed in Poland in 1989, Tusk co-founded the Liberal Democratic Congress, a party of staunch economic liberals, which went on to win nearly 8 percent of the vote in the 1991 parliamentary elections, ushering the then-34-year-old Tusk into parliament for the first time. But in snap elections held just two years later, the party failed to make it into parliament and disappeared from the political scene.

At that time, few expected Tusk to go far in politics. He was widely considered to be rather averse to hard work, a characteristic he once acknowledged. “I have a natural inclination, which I have perhaps cultivated too well, to avoid situations that involve effort or doing unpleasant things,” he stated in an interview with the Polish magazine Viva. Nevertheless, Tusk clung to the margins of Polish politics until 2001, when he co-founded Civic Platform. In the interim, Tusk had learned to appreciate the importance of popular appeal in democratic politics. In a 2001 interview, he even admitted that he didn’t consider populism a “sin but rather a political virtue,” adding, “There is no party in the world that will go against what people want against its own popularity.”

Tusk’s version of populism, however, was still steeped in liberal economics, with Civic Platform calling for a 15 percent flat tax rate and a “cheap state” with less bureaucracy and spending. But despite his best efforts, Tusk had trouble shaking his old image; “boring” and “uninspiring” were the most typical adjectives ascribed to him during his unsuccessful 2005 presidential campaign against the conservative Lech Kaczynski (who later died in the Smolensk plane catastrophe in 2010).

Tusk’s breakthrough came in October 2007, two years after his failed presidential run. Tusk was running, under the Civic Platform banner, to replace the country’s self-assured prime minister, Jaroslaw Kaczynski—then-President Lech Kaczynski’s twin brother and leader of the conservative Law and Justice party. Expectations were not high before the one-off live televised debate. Law and Justice had a solid lead in the polls, and after an unimpressive campaign, even Tusk’s supporters had mostly given up the fight. His career seemed on the precipice of collapse.

But Tusk displayed a killer instinct perfectly suited to the moment. In a relaxed and friendly manner, he fired off well-rehearsed common-sense arguments relatable to everyday Poles. (The ill-prepared 58-year-old Kaczynski, by contrast, came across as arrogant and out of touch.) Tusk’s preparations for the debate also displayed a newfound ruthlessness and readiness to fight dirty. His campaign team stacked much of the studio audience with his supporters who, in clearly premeditated fashion, cheered and clapped loudly for Tusk while bursting into jeering laughter whenever Kaczynski spoke. Over the course of the debate, Kaczynski grew increasingly irritable, while Tusk’s confidence grew in proportion to his smile. By the end of that night, Tusk was the most popular politician in Poland. He rode the momentum to victory in the election.

Over the next seven years as prime minister, Tusk carefully maintained his friendly disposition in public. He spent most of his early years as prime minister successfully steering the country through the international financial crisis; Poland was the only EU nation to avoid recession in 2009, a fact that earned the praise of global economic observers. In Tusk’s seven years running the government, Poland witnessed steady economic growth, and Tusk remained popular with the public.

But he didn’t hide his ruthless streak in private, ruling Civic Platform with an iron fist. Over time, he forced all potential rivals out of the party. Occasionally, he engineered harsh disciplinary actions after minor misdemeanors, as in the case of Zyta Gilowska, a charismatic deputy leader of Civic Platform who was forced to leave the party after accusations of nepotism. Other times, he subjected potential rivals to humiliating marginalization until they eventually quit on their own.

Tusk’s success at eliminating intraparty challengers had unintended consequences: Civic Platform has been hobbled by a lack of strong leadership since Tusk’s colleagues in Europe tapped him in 2014 to depart for Brussels to run the European Council, thus clearing a path for Law and Justice to return to power one year later. And yet, given his visibility and relative popularity, Tusk, from his perch in Brussels, remains the de facto opposition leader in Warsaw.

Since becoming president of the European Council, the conciliatory public version of Tusk has most often been on display. After taking some time to learn the ropes of Brussels’s politics, he has spent most of his tenure trying to rally Europe’s leaders around a constructive solution to the migrant crisis that erupted in 2015 and to deal with the fallout from the Brexit vote.

But the hard-nosed private Tusk has surfaced in his communications with Trump. A case in point was Tusk’s reaction to Trump’s admission in July that he considers the European Union a “foe” due to its trade rules. Tusk responded by tweeting a sort of taunt: “America and the EU are best friends. Whoever says we are foes is spreading fake news.”

Tusk, unlike Europe’s many consensus-oriented establishment politicians, has a natural affinity for Trump’s style of adversarial politics. The European Council president has developed a theory for the best way to communicate with Trump, saying to the Polish TV station TVN24 in June that the U.S. president “likes and respects those who say things bluntly, in simple phrases.” In practice, Tusk’s method has involved mirroring Trump’s often superlative, and derisive, language back at him in a manner that is indirect and disarming yet firm.

The confrontational relationship between Tusk and Trump, however, is ultimately rooted in their deep disagreements on policy. Trump’s natural ideological ally is Tusk’s domestic rival, Law and Justice. The U.S. president shares its negative attitude toward immigration (especially from majority Muslim countries), wariness toward Germany, impatience with an independent judiciary, and hostility to EU institutions and international law. (It’s no accident that when Trump visited Warsaw in 2017, Law and Justice pulled out all the stops for him, including busing in party supporters from all over the country to cheer during his speech.)

Tusk, a committed liberal and a passionate advocate for the EU throughout his career, is on the opposite side of the ideological spectrum. He believes former Soviet bloc nations such as Poland have no choice but to firmly entrench themselves in Western institutions. In a July interview on Polish TV, Tusk voiced concern that rapprochement between the United States and Russia would leave Eastern European countries such as Poland—possessing neither nuclear weapons nor significant wealth—vulnerable to bullying by Moscow. This distinctively Polish perspective, stemming from the country’s World War II experience of Soviet invasion followed by nearly a half-century of communist rule directed from Moscow, informs Tusk’s worries about Trump.

“I’m warning those in Europe who are happy to see that Trump can punch Europe” that a rebalancing of strategic U.S. relationships “should be a source of concern in our part of the continent, including in Poland,” he told the Polish TV station in July. Many Poles believe that Tusk’s geopolitical concern for his native country makes it inevitable that he will return to Polish politics once his European Council term runs out in November 2019.

Tusk has given plenty of signs that he hopes to re-enter Polish politics—often while indulging his talent for jaunty rhetoric. In July, he declared that, were Jaroslaw Kaczynski, who still leads the Law and Justice party and by all accounts its government (though he has no official position in it), to run for president in 2020, “I would definitely not hesitate for one moment to do battle with him. Let’s see if he accepts the challenge.” Tusk knows Kaczynski has already stated his party will back the popular and folksy incumbent Andrzej Duda for a second term. The announcement seemed Tusk’s way of getting under Kaczynski’s skin while reminding Poles that there are only two political heavyweights in the country—himself and Kaczynski, whom he regularly bested before leaving for Brussels.

If Tusk does challenge Duda in 2020, polls suggest that he would have a good, though by no means guaranteed, chance of winning. (Today, 51 percent of Poles, based on a poll conducted in June, say they would back Duda’s re-election, but Tusk recently overtook Duda as the “most trusted” Polish politician.) A separate question concerns whether Tusk, who has tasted real power as Poland’s prime minister, would even want to serve as the country’s largely symbolic president. The answer may turn on the presidency’s veto power: Although Tusk wouldn’t have the power to enact his own domestic agenda, he would have significant influence to block Law and Justice’s.

Tusk has no access to that power in Brussels. Last year, the European Commission, with support from Tusk, triggered the so-called “nuclear option” procedure against Poland, stipulated in Article 7 of the EU’s constitutional treaty, over measures taken by the country’s ruling party that allegedly undermined the independence of Poland’s judiciary. The process could eventually strip Poland of its EU voting rights. But Kaczynski has remained undeterred.

It’s clear that Tusk is eager to enter the fray wherever right-wing populists increasingly pose a challenge to European liberalism. But it’s likely that he will soon have to choose his battles. For now, Tusk is focused on facing down Trump’s erratic economic and military threats to the entire continent. He may soon decide, however, that he’s more needed at home.