Historically, the division of Jews in many Western countries into denominations, which in the United States in particular took the form of three large groups known as Orthodox, Conservative and Reform, can be traced to Jewish reaction to the Haskalah (Jewish Enlightenment) and its aftermath, and to a certain extent the philosophies of these movements were shaped in reaction to one another. Several smaller movements have emerged in the years since. In more recent years, all of these movements have been shaped by the challenge of assimilation.