Little more than a decade after joining the SPD in 1977, Sigmar Gabriel was voted into the parliament of Lower Saxony, his home state. He was elected state premier in 1999, remaining in the post until 2003. Gabriel's success didn't go unnoticed by the SPD leadership. Later that year he was appointed the party's "Representative for Pop Culture and Pop Discourse," earning the nickname "Siggi Pop."

As Gabriel rose through the ranks, his political ascent closely mirrored that of then-Chancellor Gerhard Schröder. Like Gabriel, Schröder also began his career in the state parliament of Lower Saxony. Given their similar backgrounds and political leanings, Gabriel was widely viewed as the chancellor's protege and potential successor.

Gabriel was appointed environment minister in Angela Merkel's first government in 2005, where the SPD was a junior partner. Gabriel made climate change the focus of his four-year tenure, leading EU negotiations on efforts to cut greenhouse emissions to 20 percent of their 1990 levels. He also accompanied Merkel on a two-day visit to Greenland on a global warming fact-finding mission.

After SPD leader Franz Müntefering resigned on the back of the party's 2009 election defeat, Gabriel elected as his successor. As leader, he withdrew the SPD from the Socialist International and laid the foundation for forming the Progressive Alliance, an organization of progressive parties whose members include the US Democrats and UK Labour Party.

Following the 2013 federal elections, Gabriel led negotiations that saw the SPD once again join Merkel's government as a junior coalition partner. As SPD chairman, Gabriel took on the role as vice chancellor and was appointed economy minister. Gabriel brought energy policy under the Ministry's remit and oversaw the country's transition away from nuclear energy towards renewables.

Following Frank-Walter Steinmeier's nomination as president in January 2017, Gabriel was picked to take over as foreign minister. Gabriel adapted into the new role and quickly became an outspoken critic of President Trump, whom he accused of "weakening" the West. He also denounced US sanctions against Russia, which hit EU energy programs such as the Nord Stream 2 gas pipeline.

Questions surrounding Gabriel's leadership rose as early as 2015 when he was re-elected party chair with just 74 percent of the vote — the lowest for an SPD leader in 20 years. With little hope of unseating Chancellor Merkel in upcoming elections, Gabriel stepped down. His proposed successor, former EU Parliament President Martin Schulz, took over as SPD chairman and chancellor candidate.

Despite suffering historic losses in the 2017 election, the SPD will still join Merkel in yet another grand coalition. After the SPD and CDU/CSU agreed on a policy blueprint, it became apparent that Gabriel would not be part of the new Cabinet. He will, however, continue to serve as a Bundestag representative for the constituency of Salzgitter-Wolfenbüttel, a seat he has held since 2005.