The ill-prepared and unfortunate Treaty of Versailles (q.v.) had left the left bank of the Rhine plus an area 50 kilometres deep on its right bank permanently demilitarized by order. This order was made again at the signing of the Treaties at Locarno in 1925. Britain and Italy (!) were to be the guarantors.
German governments since 1918/19 had wished to terminate the demilitarization, for the natural reason that it decreased German authority and, worse, exposed the very centre of German industry (the Ruhr) to a possible French attack.
Almost as soon as he was made Chancellor in 1933 Hitler said he intended to deal with the situation in the Rhineland, and had in fact planned an assault for 1937. He brought this date forward to March of 1936 to take advantage of the fact that the other European powers were distracted by Italy’s invasion of Ethiopia. Italy, you will remember, was one of the guarantors of the demilitarization. First, Hitler offered non-aggression pacts to France and Belgium and other Eastern European countries. The French trade unions, press and almost all political parties despised the idea of another war so soon, and were not against the idea. The British General Staff, worn to shreds by the Great War, casually thought the Germans would be moving into their own back garden and decided to do nothing.
German re-occupation of the Rhineland therefore took place. Hitler said later that if the huge French army had counter-invaded, the Germans would have been forced to withdraw, but this was palpably one of his lies as the German army of re-occupation was under strict orders from the Fuehrer to withstand any attempt to dislodge them from their Rhineland.
Germany’s taking back of the Rhineland has been seen by historians as an essential step for Germany, and a crucial one leading to the Second World War. Its success certainly emboldened and encouraged Adolf Hitler. It also showed French lack of will and determination to fight, and this frightened other European countries and left the Little Entente, an alliance between Czechoslovakia, Romania and Yugoslavia established by the Treaties of St. Germain and Trianon, 1919 and 1920 respectively – in tatters. These countries and others wondered if they should not come to terms with the Nazis, as France had shown clearly that she would not honour her pledges.