A 16th-century English proverb holds that "turnabout is fair play." That may be true for election campaigns, sporting contests, business disputes, even war. But it should have no place in the objective rule of law. Turkish Prime Minister Recep Tayyip Erdogan's five-year-long campaign of investigation and imprisonment of alleged coup plotters is a case in point.

In 2007, a cache of explosives was allegedly found at the home of a former military officer. State prosecutors tied the find to "Ergenekon" - what they claimed was a shadowy network of ultra-national secularists in the military and national security establishment intent on overthrowing Erdogan and his ruling Justice and Development Party (AKP). At the time, the charge was not thought to be extreme.

Between 1960 and 1997, the Turkish military toppled four popularly elected governments. Its rationale for doing so dated back to the birth of the modern secular Turkish Republic, which Kemal Atatürk established in 1923. Coming out of the ashes of the failed Ottoman Empire, Atatürk believed that the only way Turkey could become a modern, independent state was to purge its Islamist tendencies in favor of a meritocratic, secular character. Turkey's generals (known by their Ottoman title, pasha) would be the guarantors of Atatürk's vision and the country's secularist constitution.

In 1997, the generals ousted the last elected government with Islamist tendencies. Two years later they jailed Erdogan, then the AKP's leader, for Islamic sedition. Then in 2001, less than two years after he ended his nine-month prison term, Erdogan was elected prime minister. With this history in mind, the existence of the Ergenekon plot seemed plausible and the arrest of more than 200 military officers for their alleged participation did not seem capricious. In fact, international observers, especially the European Union, praised Erdogan's actions as a pro-democracy move intended to preserve constitutional government.

But the Ergenekon investigation branched out in ways that suggest Erdogan is less interested in preventing a coup than in persecuting political opponents. Last year, another 200 military officers were arrested and charged with participation in "Operation Sledgehammer" - a plot to engineer a campaign of terrorist attacks as a pretext for removing the Erdogan government. The heads of the army, navy, and air force resigned in protest as 365 serving and retired military officers were arrested. Last week, Gen. Ilker Basbug, who in 2010 resigned as the chief of the general staff, was also charged with being a part of Sledgehammer.

Basbug's arrest speaks to Erdogan's expansive definition of "coup plotting." The former head of the second-largest military in the NATO alliance is accused of sponsoring websites dedicated to spreading accusations against the AKP (that they are guilty of official corruption and are linked to extremist and Islamist groups abroad).

After his arrest, Basbug told reporters: "If I am being accused of bringing down the government with a couple of press statements and one or two Internet stories, this is very hard to swallow. . . . If I had such evil intentions, as the commander of a 700,000-strong force I could have found other ways of doing it."

Critics of the Ergenekon and Sledgehammer investigations say Erdogan has used the military's past actions as a pretext to silence and imprison government critics. They note that Erdogan has placed more journalists in prison (over 100) than China has - all on charges of supporting terrorism. Among the hundreds of former and active military officials, prominent academics, journalists, and lawyers who have been detained, many are still awaiting trial, some have not even been charged, and the evidence has been roundly criticized as flimsy and even counterfeit.

Last week, 40 Sledgehammer defendants launched a court protest wearing black T-shirts with the slogan: "Justice against science, we want a fair trial" - suggesting that judges had ignored proof of government forgery and fabrication of evidence. An additional 70 defendants failed to appear and their lawyers were also absent in protest of what they see as an unfair trial. The scene prompted the European Commission on Friday to express concerns "about judicial proceedings that might put at risk the rights of the defendant. . . [and] might raise questions in the public about their legitimacy."

Erdogan's supporters reject the criticism, noting that over the years the generals have done worse. The "they're just getting some of their own coming back at them" mind-set is informed by the "Turnabout is fair play" standard, not by objective justice. It makes Erdogan look like a pasha - albeit an Islamist one.