Mr. Westerwelle in 1986. (Heribert Proepper/AP)

Guido Westerwelle, a former German foreign minister and head of the Free Democratic Party who helped Chancellor Angela Merkel win backing for bailouts for Greece, died March 18 in Cologne, Germany. He was 54.

The cause was complications from leukemia, the Berlin-based foundation he helped start said on its Facebook page.

A native of the western Rhineland region that is often cited as the cradle of Germany’s post-World War II “social market economy,” Mr. Westerwelle led his pro-market party to its best German election result in more than 60 years in 2009.

That triumph lifted the FDP into a coalition with Merkel’s Christian Democrat-led bloc as junior partner, making Mr. Westerwelle deputy chancellor. He also took the post of foreign minister.

At his first major post-election news conference, Mr. Westerwelle caused a stir by refusing to take a question in English by a British Broadcasting Corp. reporter. “Just as it’s customary in Britain that you naturally speak in English, it’s customary in Germany to speak German,” the deputy chancellor said with a smile. An interpreter then put the question to Mr. Westerwelle in German.

Mr. Westerwelle was vice chancellor between 2009 and 2011, during Merkel’s second term, and remained foreign minister until 2013.

During his tenure, he prevailed over lawmakers in his party who opposed bailing out Greece during Europe’s debt crisis. He pushed for Germany to stay out of military strikes on Libya in 2011, joining Russia and China to abstain on a vote by the United Nations Security Council backing a no-fly zone.

A lawyer by training, Mr. Westerwelle was party chairman when the FDP won 14.6 percent of the vote in national elections in September 2009.

The result allowed Merkel at the time to dump the Social Democrats as coalition partner and ally with the FDP, which had spent 11 years in opposition.

Mr. Westerwelle stepped down as FDP head and as vice chancellor in April 2011, saying that the party needed a new start after it dropped out of two state parliaments.

In Germany’s last federal election in 2013, the FDP’s unfulfilled promises of tax cuts led voters to flee the party and it failed to win seats in the lower house for the first time since World War II.

Mr. Westerwelle, whose Free Democrat predecessors include Hans-Dietrich Genscher, the foreign minister who helped negotiate the reunification of East and West Germany in 1990, was out of a job after the loss.

He co-founded the nonprofit Westerwelle Foundation for International Understanding in Berlin in 2014 “to strengthen democracy by promoting economic development worldwide.”

Mr. Westerwelle was born in Bad Honnef, near the former West German capital of Bonn, on Dec. 27, 1961. He studied law in Bonn at the city’s university.

He revealed publicly in 2004 that he was gay. He married his partner, Michael Mronz, in 2010.

— Bloomberg News