'Grün' (i) was a German unrealised plan to invade Czechoslovakia as part of a two/three-front war with Czechoslovakia, France in 'Rot' (i) and Poland in 'Blau' (i) (late 1937/21 October 1938).

Like 'Rot' (i), 'Grün' (i) was prepared under the supervision of Generalmajor Erich von Manstein, head of the operations department of the army general staff, and was based on the concept that even if 'Rot' (i) succeeded in halting a French offensive into Germany without heavy German losses, it was nonetheless a poor guarantee for German security. German security would then have to depend on the Czechoslovaks making the military mistake of remaining on the defensive, instead of exploiting their local superiority, of approximately five to one: the Czechoslovak army could put 30 infantry divisions into the field in time of war, whereas the German army could raise just 21 infantry divisions, of which five plus the Grenzschutz (border defence force) were available for the entire Eastern Front, to make a quick thrust over the 120-mile (195-km) distance to Berlin.

Therefore General Ludwig Beck, the head of the German general staff, was persuaded to authorise the preparation of another plan as 'Grün' (i) in 1937. This was based on the concept of a pre-emptive German blow at the Czechoslovak army before the French offensive had achieved decisive success. Despite the fact that this plan was prepared within the general staff, Beck continued to regard it as a military absurdity, as it would bring both the UK and France together against Germany just as the Schlieffen plan’s violation of Belgian neutrality had done in 1914.

'Grün' (i) was to begin with the deployment of the greater part of the German army in the south-eastern part of Germany. An attack was to be pressed home from Bavaria, Saxony and Silesia on converging axes. While this was taking place, a screen was to impede the French advance, and the area of Germany lying west of the Rhine river was to be sacrificed on a temporary basis. As soon as possible after the defeat of Czechoslovakia, the bulk of the German army was then to be transferred to the west, in order to fight the decisive battle of the war against France.

The planning of 'Grün' (i) incorporated a significant element of German psychological warfare both inside Czechoslovakia and internationally against Czechoslovakia’s allies. At the internal level, the Czechoslovak government and citizenry were to be intimidated and have their will to defend themselves broken, while the ethnically German minority, which was vastly pro-Nazi, was supposed to weaken and disrupt the country. At the international level, co-ordinated Nazi psychological and propaganda warfare was directed to rendering Czechoslovakia isolated to the point at which it was faced with the prospect of having to stand alone against any aggression without effective defence. The media, especially radio, played major role in the psychological warfare effort and, inside Czechoslovakia, Germany also placed considerable reliance on the effective use of the Sudeten German Party as well as its paramilitary organisation, the Freiwilliger Schutzdienst.

On 17 September 1938 Adolf Hitler ordered the establishment of Sudetendeutsches Freikorps, a paramilitary organisation which took over the structure of Freiwilliger Schutzdienst/Ordnersgruppe, an organisation of ethnic Germans in Czechoslovakia which had been dissolved by the Czechoslovak authorities during the previous day as a result of its implication in terrorist activities. The organisation was sheltered, trained and equipped by German authorities, and was thus able to undertake cross-border terrorist operations into Czechoslovak territory.

The basic German plan was revisited and revised as the military situation and requirements changed late in 1937 and into the third quarter of 1938. The ultimate revision of the plan scheduled the attack for 28 September 1938 but, as France and the UK were unwilling to go to war for the sake of Czechoslovakia and expressed political will to avoid such a war at all costs, the execution of the plan was postponed, then, after the Munich Conference (also called the Treaty of Munich) held on 30 September 1938, abandoned altogether.

In its forced cession of the Sudetenland to Germany, Czechoslovakia lost the majority of its border fortifications and was no longer defensible in any realistic form against the German military. Germany invaded Czechoslovakia on 15 March 1939 in 'Südost' encountered minimal resistance, and annexed Bohemia and Moravia to the Reich. Germany gave nominal independence to Slovakia and installed Jozef Tiso as head of a satellite state.