The JBTZ trial or the JBTZ affair (Slovene: afera JBTZ), also known as the Ljubljana trial (ljubljanski proces) or the Trial against the Four (proces proti četverici) was a political trial held in a military court in Slovenia, then part of Yugoslavia in 1988. The defendants, Janez Janša, Ivan Borštner, David Tasić and Franci Zavrl, were sentenced to between six months' and four years' imprisonment for "betraying military secrets", after being involved in writing and publishing articles critical of the Yugoslav People's Army. The trial sparked great uproar in Slovenia, and was an important event for the organization and development of the democratic opposition in the republic. The Committee for the Defence of Human Rights was founded on the same day of the arrest, which is generally considered as the beginning of the so-called Slovenian Spring.
In the late 1980s, Slovenia embarked on a process of democratic reform, which went unparalleled in the other five Yugoslav republics. The Slovenian communist leadership, under Milan Kučan, was allowing an ever-greater degree of freedom of the press. The magazine Mladina was taking advantage of this and became extremely popular in Slovenia, deliberately testing the borders of press freedom with news and satire breaking old taboos. In 1987 it started more and more frequently attacking the Yugoslav People's Army (YPA) and its leadership, for instance labeling the defense minister, Branko Mamula, a "merchant of death" for selling arms to famine-stricken Ethiopia. Many of the articles were written by the young defense expert Janez Janša, who soon became a particular irritant for the YPA leadership. As far as the YPA were concerned, Mladina was attacking the army, the main protector of Yugoslav unity, and hence attacking Yugoslavia itself. When they realized that the Slovene government were not going to crack down on Mladina, they decided to do so themselves.