Not long after German Chancellor Angela Merkel's third term began, she buttonholed Andrea Nahles following a cabinet meeting in the Chancellery. Nahles was labor minister at the time, part of the coalition government pairing Merkel's conservative Christian Democrats (CDU) and Nahles' center-left Social Democrats (SPD) that had been in office for just a few months, and Merkel wanted to do Nahles a favor. Pope Francis was preparing to canonize two of his papal predecessors and the chancellor asked Nahles if she wanted to represent the German government at the ceremony. It would be good, Merkel said, if a Catholic were to attend the event rather than a Protestant such as herself.

Nahles was surprised by the offer, but pleased. As a child, she had been an altar girl and when she became a teenager, turned into a devotee of Pope John Paul II. Some SPD members are suspicious of Nahles' faith, but Merkel, the daughter of a Protestant pastor, respects her religious beliefs. A few days later, Nahles found herself listening to Pope Francis on St. Peter's Square as Merkel's ambassador, surrounded by dignitaries from around the world, with a large cross around her neck.

The episode took place four years ago, but it says a lot about the relationship between the two women -- one that has become crucial for the new governing coalition that Merkel now leads . Nahles, after all, is no longer a cabinet member. In April, she will become head of the SPD -- and it will be up to her and Merkel to make this coalition work.

It won't be easy: Many in the SPD were opposed to joining the conservatives in a new government and would prefer to be in the opposition. Many within Merkel's Christian Democrats (CDU), meanwhile, have begun looking past the chancellor to the post-Merkel era. And there is a widespread view in the country at large that the alliance has become outmoded before it has really even got started . But it is up to the two of them to make sure this coalition, which was assembled with such great difficulty  over the course of the past several weeks, doesn't immediately come tumbling down.

Over the years, Merkel and Nahles have developed a close, if not always harmonious relationship. It's something of a miracle they can stand each other at all, given that they seem to come from two, completely different planets. Merkel, the control freak, and Nahles, the impulsive politician. Merkel cut her political teeth under the dour Helmut Kohl, while Nahles was once called a "gift from God" by Oskar Lafontaine, the narcissistic leftist who once led the SPD.

Thrown Together

As is so often the case in Berlin, the two were thrown together by political calculation. Years ago, Nahles realized that a closer bond with Merkel would help her shed the reputation she had acquired as a leftist ideologue. And Merkel recognized early on that Nahles could help solidify her hold on power. All the way back in 2008, she even said that she should call Nahles instead of Kurt Beck, the embattled SPD leader at the time, if she wanted to arrange something with the party.

From now on, it will be up to Merkel and Nahles to iron out differences between the two parties. Sigmar Gabriel, who was Merkel's vice chancellor for the past several years, has been relegated to the role of backbencher by Nahles. And the new vice chancellor, Finance Minister Olaf Scholz, doesn't have nearly as much support within the SPD as she does.

Merkel, in other words, is partly dependent on Nahles. But the incoming SPD head is in the awkward position of serving two masters. While she will play a key role in ensuring that Germany's new coalition government doesn't meet an early demise, she must also help her party regain the confidence of voters and revitalize its center-left profile. Is such a thing possible?

What's certain is that, if necessary, Nahles can go on the attack. During the SPD party conference in Bonn in January, Nahles was on stage as party delegates debated whether to join Merkel's conservatives in yet another coalition. Nahles was beginning to get annoyed by the skepticism shown by many of the delegates. Finally, she let loose. Of course the SPD has to win back trust and develop new ideas in the next several years, she said. "But what in the world does that have to do with Merkel and the rest?" The problem, she argued, was entirely within the party itself.

Nahles decided early on not to become a cabinet member this time around, though she could have if she had wanted to. She had spent years heading up the labor portfolio and she is well aware of her party's need to once again engage in political debate, not just internally but also, and perhaps especially, with Merkel's conservatives. Sigmar Gabriel was always viewed within the SPD with a certain amount of distrust for several reasons, but one of them was the suspicion that he was too close to Merkel.

A Strident Party Shill

Nahles is fond of remembering some of the more insulting verbal attacks she fired at Merkel early in her career, particularly during campaign season. They were typical political barbs, but Merkel -- who has never been known for being particularly precious about such things -- would always send an emissary to Nahles to complain.

Things changed with the beginning of the second coalition between Merkel and the SPD in December 2013. Nahles gave up the office of SPD general secretary and joined the cabinet, partly a calculated move to shed the image of a strident party shill. Nahles burrowed into the file folders and left the SPD to party head Gabriel.

While it was clear that Merkel was the boss, Nahles held one of the most important portfolios and she was also responsible for implementing almost all of the SPD's dearest projects: lowering the retirement age to 63; introducing Germany's first national minimum wage; and restructuring laws pertaining to temporary jobs. From the very beginning, Nahles made sure to keep Merkel abreast of developments. Merkel never learned of a new Labor Ministry proposal from the newspaper, as would occasionally happen with Gabriel.

Nahles and Merkel would coordinate over the phone, they would meet for face-to-face talks on the sidelines of coalition meetings and would chat in the Bundestag. When Gabriel wasn't there, Nahles would sit next to Merkel in cabinet meetings. "Discipline, tenaciousness and persistence," those are the most important qualities for a politician to have, Nahles once said. Merkel, who likewise doesn't shy away from diving into policy details, agrees.

Plus, Merkel admires the fact that Nahles isn't easily intimidated. Over the last several years, the Chancellery has watched the SPD closely to see who is given free rein and who is not. With very few exceptions, Gabriel always refrained from constraining Nahles in front of the conservatives. But when Gabriel forced then-Justice Minister Heiko Maas to backpedal on data retention policy in spring 2015, it was clear to everyone that when push came to shove, Maas had to fall into line.

Nahles and Merkel share an unerring nose for political power, the most recent example of which was Nahles's choice of the obedient Maas as foreign minister (one of the cabinet positions that had been allotted to the SPD). The holder of the Foreign Ministry portfolio can typically expect a significant boost in popularity and Nahles was wary of doing such a favor for potential party rival Katarina Barley, who was made justice minister instead. Similarly, Merkel promoted Annegret Kramp-Karrenbauer  to the position of CDU general secretary  in an effort to quiet her critics from the party's conservative wing.

Unceasing Annoyance

The two women, to be sure, have had their share of conflicts. At a cabinet retreat in 2016, Nahles presented a set of proposals focusing on tech innovation in the working world. The project wasn't outlined in the coalition agreement, but Nahles felt that it was an issue the government needed to address.

Merkel listened, but she didn't throw her support behind the labor minister, believing Nahles was getting ahead of herself. Who knows, after all, how the corporate world would tackle things on its own? It was, in short, a clash of two different views of politics. Nahles seeks to exert control over things while Merkel prefers to sit back and observe. And the chancellor's approach is a source of unceasing annoyance to Nahles.

That wasn't the only conflict. Nahles was skeptical of Merkel's refugee policies from the very beginning and didn't join Gabriel in enthusiastically embracing the chancellor's initially open-armed welcome to the newcomers. She was more concerned about the potential integration hurdles she saw for the country and the labor market.

But Nahles kept quiet when it came time to implement conservative pet projects, such as increasing the so-called "mother's pension," a controversial state-supported payout to women who chose to stay home with their children. In exchange, the chancellor supported her when Nahles angered the German business community with her January 2014 pension package.

Both Merkel and Nahles initially got to know their parties as bastions of male dominance, but managed to break through barriers -- an element of their biographies that binds them together. Merkel managed to seize leadership of her party by writing an op-ed in the Frankfurter Allgemeine Zeitung in 1999, in which she publicly berated ex-Chancellor Kohl for his role in the CDU donations scandal. In doing so, she also distanced herself from then-CDU head (and present-day Bundestag president) Wolfgang Schäuble, who had remained loyal to Kohl. The move ultimately propelled her to the very top of the CDU, and won her a reputation for ruthlessly pursuing her own interests.

Expressions of Feminine Power

Nahles also has to live with a reputation as a cold-blooded, power-hungry politician. Many in the SPD still recall an incident in 1995 when a young Nahles, who was head of the party's youth wing at the time, screamed in excitement when Oskar Lafontaine toppled then-party head Rudolf Scharping. Ten years later, it was Franz Müntefering's turn: He resigned when Nahles ran for SPD general secretary against his will and emerged victorious. She ultimately declined to take the position, but has nevertheless been seen since then as a woman unable to tame her ambition - a cliché that Merkel has likewise been unable to shed.

It took many years for Merkel to find her place as a woman in political power, to discover suitable expressions of feminine power. Her appearance was long a topic of discussion - for much longer than it ever would have been for a man. Her first boss in politics was Lothar de Maizière, who was the last prime minister of East Germany and the only one to have been elected democratically. De Maizière appointed Merkel as his deputy spokesperson in 1990, but while he may have helped launch her political career, he also used to make fun of her floor-length skirts. Later, after Merkel had become head of the CDU, she triggered a mini-scandal when she visited Palestinian President Yasser Arafat in the Gaza Strip while wearing a short skirt.

When Merkel became chancellor in 2005, she adopted a kind of uniform: black trousers and a blazer. From then on, her first task of the day involved heading up to the top floor of the Chancellery where her personal stylist, Petra Keller, was waiting to apply Merkel's makeup and do her hair.

Nahles, too, has dressed for a variety of different roles: the casual look, in sports shoes; a more severe look, in pantsuits. She has even made the occasional appearance in a dirndl, the traditional Bavarian dress. Today, she has her curly hair blow-dried into gentle waves ahead of important events. And she still finds it unfair that loud women are seen differently than loud men. Years ago, she hired a coach to help her navigate that prejudice. "Women in public have to invest more in their voices," she once complained. But she has remained strident nonetheless.

A Hopeless Amateur

And that doesn't bother Merkel. When Nahles became SPD floor leader after last September's election, she immediately attracted a bit of negative media attention for saying that from then on, conservatives in parliament were going to "get popped in the mouth." Merkel, though, wasn't particularly bothered. After all, she can be a bit uncouth herself at times, though never when the cameras are trained on her. Plus, she has to get along with Nahles for the next few years. If the most important axis in the government isn't strong, then everything else is lost - that, at least, has been Merkel's experience in the last 12 years.

Her first coalition with the SPD, which began in 2005, started out well thanks to her good relationship with Franz Müntefering. Her next coalition was with the business-friendly Free Democrats (FDP), and Merkel respected the party's leader, Guido Westerwelle. But only a short time after the cabinet took office in 2009, Westerwelle was pushed out as FDP boss and replaced by Philipp Rösler. And Merkel never had much use for Rösler, seeing him as a hopeless amateur, like large swaths of the party he led.

The biggest challenge facing Merkel and Nahles is the fact that, now that they are paired in a governing coalition, their interests are divergent. Merkel's primary goal is to protect Germany from the political upheaval facing many of its neighbors. She wants to ensure the CDU's survival as a big-tent party and find a successor that will protect her legacy, which is why she brought Kramp-Karrenbauer to Berlin.

If Merkel wants to decide herself when she leaves the Chancellery, she has to ensure that her new coalition doesn't disintegrate prematurely, which could sweep Merkel from office. It is also in Nahles' interest that the coalition does well initially. The SPD is currently much too weak for a premature election campaign. On the other hand, she has to demonstrate that her party isn't just there to rubber stamp Merkel's legislative priorities. Each new law proposed by the government is to be carefully examined by SPD lawmakers: Nahles, after all, sees herself more as a corrective than as a Merkel toadie. Furthermore, Nahles has watched as the conservative wing of the CDU has sought recently to drag the party back toward the right - and views that development as an opportunity for the SPD.