The Brandenburg Gate is seen with a rainbow flag projected onto it during a vigil for victims of a shooting at a gay nightclub in Orlando nearly a week earlier, in front of the U.S. Embassy in Berlin on June 18, 2016. (Adam Berry/Getty Images)

After years of defending her party’s opposition to same-sex marriage, German Chancellor Angela Merkel stunned many by announcing this week that members of her center-right, governing Christian Democratic Union (CDU) party would no longer be asked to toe the party line. Instead, she would let parliamentarians vote “according to conscience.”

In the current Bundestag, a conscience vote — without wrangling by the party whip — is widely believed to be an almost certain win for equal marriage. Parliamentarians, including those of the CDU, responded speedily, calling for a vote as early as this Friday.

Why now? Domestic and international politics explain this development.

A little history in Germany

Germany was home to the first homosexual rights movement in the late 1800s and was well-known as a bastion of sexual liberalism during the Weimar Republic. But since World War II, it has been slower than many other developed nations to recognize gay rights. In 2001, Germany’s center-left Social Democratic Party (SPD) and Green Party coalition government introduced registered partnerships for same-sex couples, but it has not as yet joined peer nations in moving to full marriage rights. By now, 13 European states grant full marriage rights and 27 offer some form of partnership recognition.

Several factors explain Merkel’s hesitation on marriage and her recent shift. In my 2009 interviews, CDU members explained their party’s reluctance by saying they needed to keep the support of a spectrum of voters to the right of center. Until recently, the CDU and its sister party, the Bavarian Christian Social Union (CSU), defined by Catholic values and support for “traditional” families, had no credible challenges on the right — and wanted to keep it that way. Such a fracturing had happened to the SPD over previous decades, as many voters defected to the comparatively new Green and Left (Linke) parties.

In recent years, a challenger did emerge to CDU’s far right: the Alternative for Germany (AFD) party. But it seems less threatening than it did this time last year. Moreover, the AFD’s co-candidate Alice Weidel is openly lesbian, and the party has used a tried political strategy called homonationalism, appealing to gay voters by painting them as part of a nation under threat by “homophobic” Muslim migrants and refugees.

The domestic calculus changed and Germans now support marriage equality

But now, according to a 2017 survey, 83 percent of Germans support full marriage rights for same-sex couples. An election is coming up in September. While that figure varies, a majority of constituents of every major party agree: CDU/CSU voters by 64 percent; SPD, 82 percent; Linke, 81 percent; Greens 95 percent; and the centrist Free Democrats (FDP) by 63 percent. Even 55 percent of AFD voters support same-sex marriage.

All of Merkel’s rival parties in parliament have responded accordingly. Three parties — the SPD, FDP and Greens — have made support for full marriage rights a requirement for entering any governing coalition. The CDU’s current coalition partner and main challenger for the chancellery, the SPD, has made same-sex marriage central to its party platform.

With less to lose to her right, Merkel therefore has a strong incentive to erase any advantage her leftward challengers might bring on marriage equality. The SPD’s charismatic candidate for chancellor, Martin Schulz, responded to Merkel’s remarks by calling for the vote this coming Fridaybreaking coalition ranks.

What changed in Germany?

Research has increasingly shown that support for same-sex unions spreads across borders. As political scientist Kelly Kollman has argued, the fact that so many states adopted same-sex unions in a short period of time “is not a mere coincidence.” My recent book shows that various channels that tie states to their respective international communities have influenced the spread of gay rights.

Some such transnational sources of influence are indirect and social, as when openly LGBT people are increasingly visible in the media. Others are direct and political; Germany is a member of various international organizations that officially support LGBT rights, including the European Union and the Council of Europe. Still others are brokered through advocacy networks, like the International Lesbian, Gay, Bisexual, Trans and Intersex Association, or ILGA.

How does that work? For instance, after former president Barack Obama announced his support of marriage rights in 2012, transnational groups protested in front of Merkel’s chancellery, calling Germany comparatively retrograde — despite the fact that it had a stronger overall LGBT rights record than did the United States.

Similar pressure arrived when Ireland passed equal marriage rights by referendum in 2015. German CDU politician Jens Spahn said, “You would think what the Irish Catholics can do, we could do, too.” Volker Beck of the German Greens was similarly blunt, asking “why Germany is living in the twentieth century on this issue and not the twenty-first.”

External voices chimed in as well. People like Luxembourg’s openly gay prime minister and the Irish “Yes” Campaign activists discussed support for marriage while visiting Germany. A 2015 German political cartoon by Jürgen Tomicek depicted a stunned Merkel doomed to catch the bouquet from a newly wed Irish couple.

In other words, pressure on Merkel’s government to introduce same-sex marriage has been mounting for years. A similar combination of domestic and international pressures is at work in other countries, including ones that are currently considered “not ready” for LGBT rights. Orthodox Greece and Catholic Italy introduced registered partnerships within a year of the Irish “Yes” vote — and might yet surprise us with marriage rights.

Phillip M. Ayoub is former Humboldt Foundation German Chancellor Fellow and an assistant professor of politics at Drexel University. He is author of “When States Come Out: Europe’s Sexual Minorities and the Politics of Visibility” (Cambridge University Press, 2016).—