Reform Judaism began in early nineteenth-century Germany among Jews who had just been offered something no European Jewish community had been offered before: citizenship. Emancipation raised a question the tradition had never had to answer, whether a Jew could be a full member of a modern nation-state and a Jew at the same time, and Reform is one answer to it.
The first changes were liturgical and aesthetic, and they look mild now: a sermon in German, an organ, a choir, decorum, shorter services, and the removal of prayers for the restoration of the Temple and the sacrificial cult. The theology followed. Abraham Geiger, the movement's leading thinker, argued that Judaism had always changed, that the rabbinic law was itself a historical product responding to its circumstances, and that the enduring content of the tradition is ethical monotheism and the moral vision of the prophets rather than the ritual code.
From this came the classical position: the ceremonial law is a human product of its age and no longer binding, while the ethical demand is binding absolutely, and Israel's vocation is to be a light to the nations wherever it lives. The American statement of it, the Pittsburgh Platform of 1885, is remarkably blunt, it rejects the dietary laws and priestly purity rules as obstructive to modern spiritual elevation, and describes Jews as a religious community rather than a nation, with no expectation of return to Palestine.
Almost all of that has since been reversed by the movement itself. The Columbus Platform of 1937 restored Jewish peoplehood and embraced Zionism; later statements restored Hebrew, ritual, and the language of commandment as a category to be engaged with rather than dismissed. Contemporary Reform is markedly more observant in practice than its nineteenth-century founders would have thought necessary or desirable.