For about 16 hours, Emmanuel Macron was the darling of the European Union. Then, the warning shots began to go up, in Brussels and in capitals across the Continent.

European leaders who rejoiced at Marcon's victory suddenly found themselves confronted with a president-elect of France with clear ideas about reforming the EU — and the political capital to put those ideas in motion. He is a new and unpredictable force: liberal, internationalist and pro-integration, for sure, but unbeholden to the traditional parties that have long swapped control of the European institutions among themselves.

Perhaps most unsettling for the silver-haired denizens of backroom dealing in Brussels is that Macron, at the age of 39, has shown no sign that he will spend the next five years simply basking in the glory of vanquishing the right-wing menace Marine Le Pen. And so the joy of victory gave way to the bite of reality: uh-oh, he actually might do something.

Jyrki Katainen, vice president of the European Commission for jobs, growth, investment and competitiveness, was among the first to send up a cautionary flare, declaring his opposition to Macron’s proposal for a “Buy European Act,” which Katainen said would distort the EU market.

In Berlin, Commission President Jean-Claude Juncker declared that "the French spend too much money and spend it on the wrong things," a clear warning to Macron to get his own fiscal house in order before tackling the taller mountain of EU finances. Meanwhile, in The Hague, Dutch Prime Minister Mark Rutte cautioned Macron against thinking that Southern Europe's fiscal problems could be mended simply by creating new institutions.

"There are also elements in the views of the new French president that we find less attractive," Rutte — who dispatched his own right-wing, anti-EU challenger Geert Wilders in national elections in March — told reporters at the Binnenhof on Monday. Rutte cited Macron's support for an EU finance minister and a common EU budget as examples of not especially appetizing campaign planks.

Joining the A-list

While the result in France is the third in a string of pro-EU triumphs, following the elections of Rutte and of Alexander Van der Bellen as president of Austria in December, Macron will catapult onto the A-list of world leaders at a moment that gives him exponentially greater stature and relevance.

The EU is now in the midst of a vigorous debate over the bloc's future, with Britain preparing to make its exit, and five pathways ahead laid out in a White Paper by Juncker and his College of Commissioners. Macron can potentially exert huge sway in this debate, particularly over the next four months while German Chancellor Angela Merkel is preoccupied with her own federal elections.

That the EU power-set viewed the French elections, rightly or wrongly, as a nothing less than a referendum on the bloc's continued existence, only adds to Macron's leverage. He will arrive in Brussels for a NATO leaders' summit meeting later this month as a conquering hero. And that creates all the more reason for a sense of alarm among the entrenched powers who cheered his win on Sunday.

"In six months, he will be a great diplomat and a warlord," Alain Lamassoure, a veteran French member of the European Parliament, predicted in a telephone interview with POLITICO.

Lamassoure said Macron lacked experience, had much to learn about EU policy and that some of his proposals seemed confused or even misguided — but predicted that his embrace of the EU would make up for all of that, giving him immediate muscle in Brussels.

"The only one among the candidates who has talked extensively about Europe, the only one who put European flags in his campaign meetings," Lamassoure said. “It is so unprecedented that nobody cares whether his views on the EU are confusing or not."

Well, not quite nobody.

Katainen's quick push-back on Marcon's "Buy European" proposal suggested that officials responsible for the EU's overall economic policy were more than a bit concerned about the French president-elect's platform on trade and fiscal issues. And Juncker's remarks, in particular, indicated that if Macron pushed too hard, too fast in Brussels, France would quickly find its own finances under a microscope.

Return of the power nexus

Elmar Brok, an influential German MEP, said Macron's first task was to improve France's economy and restore the country to a leadership position in the EU.

"France has to become competitive," Brok told Deutsche Welle's English-language TV. "A good stable budget and competitiveness in legislation and investments are the three points you need for a successful economy and here I think Macron will hopefully have the strength to do that where Sarkozy and Hollande failed."

Brok added that Berlin was looking forward to once again forming a power nexus with Paris.

"Germany cannot run and wants not to run Europe alone," Brok said. "Europe was always in its best shape when it was a joint cooperation with France and Germany. We had also in the last years this cooperation but France had, because of its internal problems, not the credibility to lead. We hope that Mr. Macron will make France so strong again in this Europe context for such a joint leadership."

At the same time, a number of German officials sought to push back against Macron's criticism of Germany's trade surplus. They included Martin Schulz, who is challenging Merkel in the September federal elections.

"Germany cannot run and wants not to run Europe alone" — MEP Elmar Brok

The true extent of Macron's political strength will not be known until after the French parliamentary elections next month. That vote will determine whether he is able to effectively move a domestic agenda and create momentum for a broader push on EU reforms, or if he would have to struggle to build coalitions to pass major legislation at home, leaving less bandwidth to dedicate to Brussels.

Presidencies in play

In the most immediate sense, Macron can shake up internal EU politics simply by deciding which political family he and his party, newly rebranded La République en Marche, will join. An affiliation with the centrist Alliance for Liberals and Democrats in Europe would immediately raise the group's claim on the presidency of either the European Commission or the European Council, which will each come vacant in 2019.

Macron will also certainly be a factor in the forthcoming decision on a president for the Eurogroup, a position that Macron has said he would like to see elevated to the status of EU finance minister. The age-old debate over truly completing the EU's fiscal and monetary union could well prove a test of Macron's mojo on the European stage. Numerous leaders have pushed for such goals previously, only to fall short.

Some of Macron's campaign promises, such as to take Poland and Hungary to task over alleged rollbacks in democratic freedoms, are likely to face resistance in the arena of realpolitik. Other proposals, such as a push for Europe-wide citizen forums, closer EU cooperation on defense and security, and especially a tough line toward the U.K. in Brexit talks are more likely to be well-received.

Some of Macron's campaign promises are likely to face resistance in the arena of realpolitik.

Still, the growing skepticism about the EU's new French wonder-boy was impossible to miss. Speaking at a forum in Brussels on the EU's future, Juncker pooh-poohed Macron's idea of citizen forums. “The tool is not very important,” Juncker said. "What is important is to have a debate on Europe."

Juncker also expressed doubts about the chances of Macron's proposal for pan-European candidate lists in the 2019 European Parliament elections, despite his own support for the concept.

“I am in favor of that idea,” Juncker said. “But I doubt that political parties are ready to commit themselves to discussing that."

Maïa de la Baume contributed reporting.