The Reform movement was originally an assimilationist movement. (This is no longer the case.) It began in 19th century Germany in response to newly granted emancipation. Philosophically, it rejected the idea of Torah as revelation, and the idea of Torah as a basis for behavioral law (halacha). It regarded the Bible as a valuable folklore, abandoned Jewish nationalism, and repudiated all religious practices that were felt to be peculiar to traditional Judaism in contradiction to secular European society. Thus praying in Hebrew and wearing a head covering were prohibited. At one point, moving Sabbath observance to Sunday was debated. It was the new behavior (rather than the philosophy) that necessitated a separate community.
The wave of Jewish immigration to America in the early 1800s consisted largely of Reform Jews. In the twentieth century, Reform philosophy and liturgical practices have grown much more traditional. Zionism is now mandatory, praying in Hebrew and covering one’s head are permitted as options, and ritual appears to be making a comeback. Reform liturgy has many features that are based on the dominant non-Jewish culture, such as organ music, mixed choirs, prayers in English, lighting flames on the Sabbath, and so on. The American Reform movement has a proud record of social activism and charity work.