KAMPALA, Uganda—There is a terrible but revealing irony about the arrest and detention in Zimbabwe of 25-year-old American Martha O’Donovan. Over the weekend, tens of thousands of the country’s citizens took to the streets of the capital, Harare, many of them brandishing placards denouncing 93-year-old President Robert Mugabe. His army is in revolt, his wife and chosen successor, 52-year-old Grace Mugabe, is no longer seen in public, and even though he’s trying to hold on, his party and countless citizens of the country that he’s ruled for 37 years clearly want the president gone, gone, gone. Back on Nov. 3,...