The Province of Westphalia was a province of the Kingdom of Prussia and the Free State of Prussia from 1815 to 1946. In turn, Prussia was the largest component state of the German Empire from 1871 to 1918, of the Weimar Republic and from 1918 to 1933, and of Nazi Germany from 1933 until 1945.

The Free State of Prussia was one of the constituent states of Germany from 1918 to 1947. The successor to the Kingdom of Prussia after the defeat of the German Empire in World War I, it continued to be the dominant state in Germany during the Weimar Republic, as it had been during the empire, even though most of Germany's post-war territorial losses in Europe had come from its lands. It was home to the federal capital Berlin and had 62% of Germany's territory and 61% of its population. Prussia changed from the authoritarian state it had been in the past and became a parliamentary democracy under its 1920 constitution. During the Weimar period it was governed almost entirely by pro-democratic parties and proved more politically stable than the Republic itself. With only brief interruptions, the Social Democratic Party (SPD) provided the Minister President. Its Ministers of the Interior, also from the SPD, pushed republican reform of the administration and police, with the result that Prussia was considered a bulwark of democracy within the Weimar Republic.