Nazi German forces re-occupied the demilitarized Rhineland, violating both the Treaty of Versailles and the Locarno Treaties that were signed after World War I.

The remilitarisation of the Rhineland began on 7 March 1936, when military forces of Nazi Germany entered the Rhineland, which directly contravened the Treaty of Versailles and the Locarno Treaties. Neither France nor Britain was prepared for a military response, so they did not act. After 1939, commentators often said that a strong military move in 1936 might have ruined the expansionist plans of Adolf Hitler, the dictator of Germany. However, recent historiography agrees that both public and elite opinion in Britain and France strongly opposed a military intervention, and neither had an army prepared to move in.

The Rhineland is a loosely defined area of western Germany along the Rhine, chiefly its middle section. It is the main industrial heartland of Germany because of its many factories, and it has historic ties to the Holy Roman Empire, Prussia, and the German Empire.

The Treaty of Versailles was a peace treaty signed on 28 June 1919. As the most important treaty of World War I, it ended the state of war between Germany and most of the Allied Powers. It was signed in the Palace of Versailles, exactly five years after the assassination of Archduke Franz Ferdinand, which led to the war. The other Central Powers on the German side signed separate treaties. Although the armistice of 11 November 1918 ended the actual fighting, and agreed certain principles and conditions including the payment of reparations, it took six months of Allied negotiations at the Paris Peace Conference to conclude the peace treaty. Germany was not allowed to participate in the negotiations before signing the treaty.

The Locarno Treaties, known collectively as the Locarno Pact, were seven post-World War I agreements negotiated amongst Germany, France, Great Britain, Belgium, Italy, Poland and Czechoslovakia in late 1925. In the main treaty, the five western European nations pledged to guarantee the inviolability of the borders between Germany and France and Germany and Belgium as defined in the Treaty of Versailles. They also promised to observe the demilitarized zone of the German Rhineland and to resolve differences peacefully under the auspices of the League of Nations. In the additional arbitration treaties with Poland and Czechoslovakia, Germany agreed to the peaceful settlement of disputes, but there was notably no guarantee of its eastern border, leaving the path open for Germany to attempt to revise the Versailles Treaty and regain territory it had lost in the east under its terms.