BERLIN—Germany’s CDU/CSU conservatives and their Social Democrat rivals were tied in Sunday’s national election, an exit poll showed, leaving open which of them will lead the next government as Angela Merkel prepares to stand down after 16 years in power. The CDU/CSU bloc won 25 percent of the vote, their weakest result in a post-war federal election and on a par with the center-left Social Democrat (SPD), the infratest poll for broadcaster ARD showed. Other exit polls showed the SPD marginally ahead. “That hurts,” CDU General Secretary Paul Ziemiak told ARD after the publication of the exit polls. Attention will now shift to informal discussions—likely with the Greens, on 15 percent, and liberal Free Democrats (FDP), on 11 percent—followed by more formal coalition negotiations, which could take months, leaving Merkel in charge in a caretaker role. After a domestic-focused election campaign, Berlin’s allies in Europe and beyond may have to …