HAMBURG, Germany—The U.S. president has accused her of ruining Germany. The Turkish president says she harbors terrorists. The Russian president, her spy agencies warn, may be about to interfere in her re-election campaign. In the coming days, German Chancellor Angela Merkelmeets all three of them

In Hamburg, her birthplace, the 62-year-old pastor’s daughter hosts the Group of 20 summit thrust into a role no German chancellor has had to navigate in the postwar era. The leader of a country that generally disdains international confrontation is now the foil to three of the world’s most polarizing heads of state. Three countries that Germany had prized as partners have, in different ways and to varying degrees, become antagonists.

“The world is turbulent,” Ms. Merkel said in a speech to parliament last week. “It has become less united.”

Germany, with its export-oriented businesses and its bloody past, long shied away from global power struggles or military engagements and instead sought to build deep ties with a variety of states. Like no other country, German officials often say, Europe’s largest economy relies on a harmonious, rules-based world order.

But at the two-day gathering in Hamburg, which officially begins Friday, global disunity that has been years in the making will become personified. Ms. Merkel will be in the middle of it, and her patient, deliberate style of diplomacy will be put to the test.