The Union of Orthodox Rabbis of the United States and Canada (UOR), often called by its Hebrew name, Agudath Harabonim or Agudas Harrabonim ("union of rabbis"), was established in 1901 in the United States and is among the oldest organizations of Orthodox rabbis which could be described as having a Haredi worldview. It had been for many years the principal group for such rabbis, though in recent years it has lost much of its former membership and influence.
The Agudath Harabonim was formed in 1902, in direct consequence to Solomon Schechter's takeover of the formerly-Orthodox Jewish Theological Seminary of America (JTS). Within 100 days of Schechter's appointment, the Union formed and issued a statement critical of JTS and Schechter. They rejected any future graduate of JTS, though "... early students who emerged from there were full-hearted for the faith of Israel and its Torah."
Agudath Harabonim was also an answer to the Orthodox Union (OU), which had formed five years earlier. There was two distinct groups within the American Orthodox rabbinate: the Europeans and the American-born.... The Americans were English-speakers, often had a secular education, and competed with Reform (and later Conservative) movements for the heart of the modern American Jew. European transplants were often Yiddish-speaking with limited English skills, trained exclusively in rabbinics, and would be termed Haredi today, and had a stronger affinity to the entire body of religious texts; they were there to maintain standards. Though there were American scholars trained in the European path, and European schools that supported secular scholarship, most rabbis belonged to one camp or the other.
To the Europeans, the OU and its later affiliated Rabbinical Council of America, were a notch down in erudition, and perhaps a bit lax in religious law. Depending on where the Americans studied and how they were brought up, their credentials were also subject to question. The European set needed a fellowship to promote its ideas and raise its political capital, and the Agudath Harabanim served that need. While not entirely antagonistic to the OU, the Agudath Harabonim clearly had a diverging agenda.