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Angela Merkel prepares to exit politics.

In a surprise move on Monday, German Chancellor Angela Merkel announced she would step down as head of the Christian Democratic Union later this year, and retire from public life after serving out her term as chancellor, which is set to expire in 2021. Merkel has been a dominant figure in German political life, and indeed European politics, for nearly two decades.

She became head of Christian Democratic Union in 2000 and chancellor in 2005. During this period, she’s been one of the pillars of centrist politics on the continent, a stalwart supporter of the European Union and NATO. Her imminent departure comes at a period when both the center right and the center left in Europe are eroding in the face of more strident political movements. She was more centrist than many in her party liked, especially on the issue of accepting refugees, which became one of her signature positions. The victory of Brexit in England has cast a shadow over the project of European unification.

Merkel currently presides over a troubled coalition government, with her center right Christian Democratic Union and Christian Social Union working in coalition with the center left Social Democratic Party. This tenuous alliance has caused the centrist parties to suffer in recent regional elections in Bavaria and Hesse, which have seen the rise of both the Greens on the left and the far right Alternative for Germany.

It’s not clear that the existing coalition government can survive. Merkel’s move might trigger the break-down of the coalition and early elections. If so, her departure will come sooner.

During the rise of President Donald Trump, Merkel has been a steady advocate for NATO even in the face of an American president who mocked the alliance. In a post-Merkel Germany, that alliance, like so many other building blocks of European stability, could start to crumble.