German Chancellor Angela Merkel’s grand coalition government was plunged into turmoil Sunday when the leader of the Social Democrats resigned abruptly under pressure after her center-left party suffered a humiliating defeat in last week’s European Parliament elections. Andrea Nahles, the first woman to head Germany’s oldest party, announced she would quit as party chairwoman, parliamentary floor leader and even give up her seat in the German parliament after her attempt to snuff out a brewing rebellion over the party’s frightening slide backfired.