In 1974, the Lutheran Church Missori Synod's Concordia Seminary in St Louis experienced a student walkout.
For many decades, two schools of thought had co-existed within the LCMS (in much the same way I suppose that both Pharisees and Saducees taught in the same synagogues in 1st century Palestine). The "conservatives" believed that the Bible was God's Word and that was that: the "moderates" believed that the Bible contains God Word and any given passage within it may or may not necessarily be infallible.
Since the LCMS is a confessional church, this situation could not continue and at the Denver convention of 1969, "conservative" Dr. J.A.O. Preus (formerly president of Concordia Seminary in Springfield, Illinois, now located in Ft Wayne, Indiana) was elected president of the Synod on a platform of resolving it.
Just two months before Dr. Preus' election, "moderate" Dr. John Tietjen had been called to be president of the seminary at St. Louis. The stage was set.
At the New Orleans Synod convention of 1973, delegates voted to investigate charges that the St. Louis seminary was teaching doctrine in contradiction to Scripture and the Lutheran confessions.
On Jan. 20, 1974, the Board of Control of the seminary suspended Dr. Tietjen. Most of the students and 45 out of 50 of the faculty walked out and established the Concordia Seminary in Exile (Hence Seminex, which was later merged with the Lutheran School of Theology in Chicago).
With only a fraction of the student body and a handful of teachers left, the St. Louis seminary survived by virtue of the assistance of it's sister institution in Springfield.
The 1975 Synod convention in Anaheim authorized Dr. Preus to remove district presidents who placed or ordained Seminex graduates, which Preus proceeded to do in four instances in April of 1976.
The "moderates" formed the Association of Evangelical Lutheran Churches on Dec. 4, 1976. 100,000 people (out of 2.7 million) and 245 congregations (out of 6,000) left the LCMS. In 1988, the AELC joined with the other "moderate" Lutheran churches to form the Evangelical Lutheran Church in America.