Summits of world leaders are usually highly choreographed affairs that leave little to chance. They are held in locations that are easily shielded from demonstrators. And policy differences are papered over by envoys behind closed doors well ahead of time. But the G20 meeting in Hamburg next week will be different. Host German Chancellor Angela Merkel has taken a high-risk gamble by choosing to hold the summit in the center of the northern port city, partly to show the world that big protests are tolerated in a healthy democracy. This has created a huge challenge for police. On the policy...