The enigma about the “fourth customer,” who appears to work on a nuclear option with utmost secrecy, has never been solved, even though a resolution appears to become ever more urgent. If one compares Pakistan’s production volumes with the production that Khan sold to his three customers beyond Pakistan’s own national needs, one finds considerable discrepancies. In other words, the “fourth customer” has received much more from Khan than just the one shipment originally intended for Libya. Khan, however, remains silent. Considering that, according to intelligence sources, Turkey is in possession of a considerable number of centrifuges of unknown origin, and considering that Khan, shortly before he was put under house arrest, had travelled to Turkey, the conclusion that Turkey is the fourth customer does not appear far-fetched.

Yet this may only be one part of the story. Khan not only delivered centrifuges to his customers, he also supplied them with blueprints for the design of nuclear weapons. The CIA uncovered such plans in Libya in 2003, which had been kept in a department store plastic bag. And in the course of investigating Saddam Hussein’s nuclear activities, the IAEA found a one-page document in 1998 that turned out to be a comprehensive offer by Khan to turn Iraq into a nuclear-armed power within three years, for the price of 150 million dollars. This offer explicitly referred to providing Iraq with all necessary components and blueprints for making nuclear weapons.

If Turkey had indeed been the “fourth customer” of the Pakistani nuclear smuggler, one must assume that the country is now in possession of all documentation necessary to build a bomb. And even if Turkey had not been the fourth customer, one must assume that, given the long cooperation on the production of centrifuges, Khan did instruct his preferred partner not just in how to use centrifuges, but also in weaponization.

Given the ambiguities surrounding the level of nuclear expertise of Turkish scientists, it remains difficult to offer clear-cut facts about the current state of Turkey’s nuclear activities. What appears worrying, however, are statements from intelligence circles about an advanced nuclear program. According to some sources, Israeli prime minister Netanyahu informed then Greek prime minister Papandreou on March 15, 2010 that Turkey could become a nuclear power any time it wanted to.

Another indirect piece of evidence for the existence of a Turkish nuclear-weapons program is Ankara’s missile program. For a long time, Turkey appeared content with developing short-range missiles with a range of up to 150 km. However, over the past years, various public statements indicate a change of course. Much publicity was given to a December 2011 statement by President Erdogan, in particular his demand to the Turkish defence industry to develop long-range missiles. While Turkish media interpreted Erdogan’s statement as a plea for intercontinental ballistic missiles, it remained unclear whether the president was really thinking in these terms. However, two months later, Turkey appears to have started developing a medium-range missile with a range of 2500 km. In 2012, Turkey tested a missile with a range of 1500 km, and it also became known that the missile with a range of 2500 km would be operational by 2015.

Even if Turkey will not be able to keep these deadlines, its intention to develop medium-range missiles is clear. This raises the question as to the strategic rationale of such weapons. The answer is fairly simple: Medium-range missiles only make sense with a nuclear payload. Thus, Turkey’s development of medium- or long-range missiles can only be explained in the context of a nuclear-weapons program. In a nutshell, Turkey’s desire to build missiles with longer ranges is a strong piece of evidence for the existence of a nuclear program.

But what are the views of Turkey’s political leadership on this issue? There are, of course, no public statements arguing the case for a national nuclear option. However, some statements can be interpreted as conditioned statements of intent. In August 2011, Turkey’s ambassador to the United States, Namik Tan, said: “We cannot tolerate that Iran obtains nuclear weapons.” This position was made more concrete two years later by President Abdullah Gül. In an interview with the journal Foreign Affairs, Gül said that “Turkey will not allow that a neighbouring country has weapons that Turkey itself does not have.” Since it should be clear by now to Turkish politicians that Iran, irrespective of the deal with the P5+1, will continue to pursue a nuclear program, there is no point anymore in conditioning one’s own nuclear work. Domestic hurdles appear low: In a 2012 poll, 54 percent of the 1500 people interviewed were in favor or Turkish nuclear weapons if Iran went nuclear.

Given these developments, it becomes clear why Turkey is a legitimate target for German intelligence. A NATO ally who appears to increasingly envision its own role as that of a nuclear-armed regional heavyweight is a development of tremendous importance that Germany cannot afford to ignore. Given Erdogan’s vision of Turkey as a self-confident, assertive and potentially independent regional leader in the Middle East, and given the existence of an established (Israel) and an emerging nuclear power (Iran), Turkey has no real alternative but to acquire nuclear arms as well. If Turkey does not opt for nuclear weapons, it will remain second class—a position that Erdogan cannot and will not accept.

Hans Rühle is a former Head of the Planning Staff in the German Ministry of Defense. He publishes frequently on security and defense matters.

Editor’s Note: This is the modified version of an article that was first published in the German newspaper “Welt am Sonntag"