Case Green was the German plan to invade Czechoslovakia in 1938. If it had been executed, WWII would probably have started a year earlier, and Germany would likely not have fared as well as she did in 1939 and 1940. The plan was an all or nothing gamble that would have pitted 37 divisions to attack the heavily fortified Czechs while leaving only 11 divisions to protect Germany from potential French and/or Polish attacks. At the time, Poland had a million-man army, while the French had nearly a million men on the Maginot Line alone, aside from their field army. Britain could quickly throw in another 200,000, with hundreds of thousands more over the following months.
Additionally, in 1938 Germany had not completed her rearmament, so her military was not yet the juggernaut unleashed on Poland in 1939 and against Western Europe in 1940. The Luftwaffe lacked the ability to strike Britain in 1938, the Kriegsmarine had few submarines, and the Wehrmacht as yet had few of the signature tanks that would spearhead the blitzkrieg in 1939 and 1940. So risky was the plan that German generals plotted to assassinate Hitler if the order to launch Fall Gruen was issued.
Unfortunately, Hitler ended up annexing Czechoslovakia without a fight because the Western Allies chose appeasement rather than confrontation, and sold out the Czechs in Munich in exchange for Hitler’s promises to behave. When Hitler broke his Munich promises and war broke out less than a year later, Germany was in a stronger position, and the Western Allies were relatively weaker. In 1938, the Allies would have fought while benefitting from the Czechs, whose well-trained and well-equipped military was hunkered behind strong fortifications and was supported with a formidable domestic armaments industry.
In 1939, the Czech military was no longer an asset in the Allies’ column, and the Czech armaments industry was churning out weapons for Germany. Indeed, a significant portion of German armaments during the war, especially early in the war, came from Czech factories. E.g.; Czech tanks such as the 38(t) played a significant role in the German conquests of Poland, Norway, and Western Europe, with 6 Panzer divisions armed with the 38(t) until 1942.